In my main campaign, I've been playing a 9th Level Life (and hopefully soon to be Twilight) Cleric of Selune. I've made this character to be a protector for the party, keeping buff spells up and tanking hits to keep our weaker members safe. I've already taken Magic Initiate for Familiar and utility spells, and I'm looking into what feats to take at levels 12 and 16 that can further my overall ability.

      I need some form of concentration protection soon, either in Warcaster or Resilient, and I'm undecided on what to take at 16th level. I've seen that at high levels proficiency with Con is better than advantage and being able to cast with both hands full isn;t a problem for me, so the question is how useful the OA spells are. 

     Our DM has confirmed that this campaign is going to go to 18th level. And I'm thinking either, Shield Master(I have low dex and the reaction Evasion is nice), Tough(I already have a +3 con, but more hp is good for tanking for the party), Elven Accuracy(more dice on advantage is great, but I don't have a reliable way to get it), or Lucky(it's just great. Super advantage on disadvantage if I use it, and I roll bad anyway) for my 16th level. 

     Any help on what would be better would be greatly appreciated. 

Double check with your DM if they allow Lucky, some don't. Are you already full up on attunement slots? Because there are some magic items that can also up your CON or give you Evasion. Have you looked at Sentinel? Good for keeping enemies close to you and away from your squishy teammates. And as a cleric you don't really have much to use your reactions on.

For Warcaster vs Resilient, there are a couple points to think about. Are you going to be using Circle of Power a lot? In that case you'll already have advantage. I'm not sure how you're handling Life vs Twilight. For the OA spells, again talk to your DM to see how they interpret the line where the spell 'must target only that creature.' Are you going to be limited only to spells like Command and Toll the Dead? Or can you use things like Flame Strike as long as the intended recipient is the only one in the area of effect? Remember also that you have disadvantage on range attacks if they're within 5 feet of you, which they would be for an OA, so if you tried Guiding Bolt you'd be at disadvantage. There are still plenty of good spells to use if you go that route, but see if Resilient and Sentinel might work better for you.

My DM allows Lucky, so that's always an option. My attunement slots are completely empty, as the campaign up to this point has been mostly about surviving and we only just now got back to civilization. And I guess I'll hold off on Tough to see what kind of defensive items I can get. I never considered Sentinel, but now that you mention it it would be useful for keeping enemies up close. 

Does the Circle of Power saving throw advantage apply to concentration? It says it gives advantage on spell effects but I'm not sure about concentration. 

I'll definitely place Resilient and Sentinel higher up on my list now. Thanks for the help.

What are your stats? That's going to make a big difference in what is optimal for your build. 

9th level, High Elf

16 str, 10 dex, 16 con, 13 Int, 20 wis, 7 cha

Dex isn't a real deficit, but it's annoying in combat when I'm in a party with a Rogue, Bard, and Monk.  

Cool, so here goes. 

You are correct, War caster is usually better early on, more specifically Tier 1 - Tier 2. Resilient catches up and exceeds WC at higher levels but having both is great if you can fit them into your feat choices. But you will need at least one to help keep your concentration and considering you've maxed wisdom already I'd say either one will work. Keep in mind that at higher levels there are many nasty con saves that can wreck your day so I generally prefer RC especially past T2. And if you had an odd Con score it would be a no brainer to just take Resilient Con. Another factor might be if you get lots of OAs where WC might be a better choice. But I'll reiterate again that concentration isn't the only thing you'll have to worry about at higher levels having Con proficiency is really nice for lots debilitating spells at any level. This a good guide for WC vs RC: https://www.google.com/amp/s/thinkdm.org/2019/12/14/war-caster-vs-resilient/amp/

Also, Circle of Power does not help for concentration saves. Here's the relevant text:
"For the duration, each friendly creature in the area (including you) has advantage on saving throws against spells and other magical effects."
Damage is still damage regardless of it's source, but you do get advantage on the save to mitigate said damage if its source is a spell attack or AOE.

Another way to mitigate damage that will result in concentration saves is to multi class into Wizard. One level will do here since what you're really after is Shield and Absorb Elements (you already have Find Familiar so pick a few more ritual spells to save on slots). Both Shield and AE are really great on a tank and since you're 1st level spells may not see much action at higher levels with all of the great higher level spells you'll have access at 9th level and beyond you'll usually have these ready to go. I'm playing a Tempest Cleric 11/ Storm Sorcerer 1 and I've gotten a lot of mileage out of both Shield and Absorb Elements. I also took Resilient Con at 8, taking my Con from 15 to 16 and I have had a pretty easy time maintaining concentration. Not getting hit and always being able to halve or quarter AOE damage in conjunction with RC (or WC) is key here.
And if you decide to go 2 levels into Wizard you can now also choose a school of magic.
The two best options IMO for a tank are:
War Magic grants you Arcane deflection where you can use your reaction to gain a +2 bonus to your AC against that attack or a +4 bonus to that saving throw and Tactical Wit which would let you add your +1 Intelligence to your Initiative, obviously better if you can increase your intelligence but every bonus counts.
Divination for portent where you roll two d20s and record the numbers rolled. You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. are really great for a tank. Diviners are really great and can turn an encounter with a single portent if you time it right, it's a really sweet feature to have in your back pocket.
Both are excellent for a tank and you still have Shield and Absorb Elements to really mitigate damage if needed.

Now on to feats...
Alert is great if you really want a better Initiative bonus, never want to be surprised again and with the added bonus that any invisible or hidden creature won't get advantage on attacks against you. Really great for the tank or any player really.
Elven Accuracy is another great feat for your attack spells or melee attacks (with a 16 Strength this not the most optimal unless you can find a +2 weapon at the very least) especially since you have a Familiar. You only get one attack despite the possibility of having Spiritual Weapon up and so making that one attack count is really nice. Have your Familiar (Owl is by far the best in this scenario) to give you advantage on your turn and then turn that Advantage into Super Advantage.
Observant is another great feat but mostly for the ability to now perceive almost anything your DM can throw at you. Your passive perception at 9th level and a 20 wisdom will be 24 which is just nuts. The only downside is that you've maxed Wisdom and you can only add it to Intelligence which may or may not help unless you multi class into Wizard where at least you'll get a +1 bump to your DC and spells prepared.
Sentinel is just ok with your main melee attack, ie 16 strength and keep in mind that the 3rd bullet point for Sentinel requires a melee weapon attack so no spells or war caster on this one. Sentinel is really good for martials that have their main attacking stat maxed for melee attacks - anybody else it's just meh.
Shield Master is actually not a bad choice here I think. You get another option for you bonus action, though as a cleric you will have lots of options depending on your spell selection that day, eg Healing Word, Sanctuary, Shield of Faith, Spiritual Weapon, etc....so your mileage may vary. But your dex is low so at higher levels you're going to have a hard time getting the 3rd bullet point to work, ie Evasion, unless you take Resilient Dex at some point. The shove action is also just ok since you're Strength is only a 16 and not maxed.
Tough never hurts and will make you a beefier tank for sure. Take this if you find yourself constantly low on hit points throughout the day.
War Caster (already discussed).
Lucky is always great since you have a reserve of 3 luck points to use on almost anything. If you can't decide on any of the above or any other feats take Lucky.

Thank you. I've spent a lot of time considered wizard (war mage for the tanks) in the past, but having spell slots not line up with spells available for my classes is just too annoying for me. Shield and AE would be incredible for my role though. 

I hadn't considered the spells and other abilities I would need con for at higher levels, so that's even more reason to take RC. Taking it at 12th for sure.

If I can get a shield with a good bonus to it, there's a good chance I'll take Shield Master, because I believe the +bonus also applies to shield master. I'll keep mental notes of how low my hp dips, each day, and whether to take Tough or not. 

I hadn't considered my Owl or Advantage, I mostly used it for cure wounds on out of range allies, but even then, most of the combat it has seen has knocked out of the sky. 

I think that now I have a concrete understanding of what feat I would want depending on my needs, I can pick the right one when I hit lvl 16. With how inconsistent our sessions have been recently, it'll be a while. 

Thanks.