I've seen analyses of the value ofResilient vs War Caster. Generally, War Caster is better at low levels and Resilient is better at high levels, I think once your proficiency bonus hits 5, but does that analysis change if/when you have Arcane Deflection from War Magic?

With the additional insurance provided by Arcane Deflection, I'm tempted to take Resilient in order to pick up the added benefit of adding proficiency to all Con saving throws, whereas the additional benefits provided by the second and third clauses of War Caster seem nearly useless without a medium armor/shield build.

Also, the free point of Con isn't too shabby.

I will admit that I'm unsure why Arcane Deflection would make a difference when it comes to the benefits of Resilient vs. War Caster. I understand that Arcane Deflection will increase your saving throw by 4, but that doesn't change which is more beneficial over all. 

Now I have a preference to take Resilient over War Caster since getting that Con Prof is beneficial in more ways than just maintaining spell concentration. That being said if your main worry is the Concentration throws, and the campaign isn't likely to go very high, then just take War Caster. The second and third points are useful outside a shield build. That ability to cast a spell as a reaction can be very powerful.

I think there are some basic points to consider:

- Do you have an odd CON score? Resilient is better.

- Are your DM forcing debilitating CON tests frequently? Resilient is better.

- Are you planning to make use of Booming Blade in your Opportunity Attacks enabled by War Caster?

- Are you having problems with Somatic only spells (Absorb Elements, etc) because you are carrying a weapon and a shield? Consider War Caster.

So the reason I ask about it with Arcane Deflection, is that it seems the ability closes the gap between how likely you'll fail a concentration save with Resilient vs. War Caster, but that means you're also more likely to be casting nothing but cantrips.for a round with Resilient. 

For example, at level 1, with 14 CON and concentration deck DC of 10: normally War Caster gives you an 87.75% chance of success and Resilient gives you a 75% chance of success. With Arcane Deflection, it's 97.75% and 95% respectively. The gap between the two is much smaller, but there's a 20% chance you're not casting spells other than cantrips for a round with Resilient and only 10% with War Caster. 

At level 5, with 14 CON but now a proficiency bonus of 3, and trying to hit DC 15: normally War Caster gives you an 64% chance of success and Resilient gives you a 55% chance of success. With Arcane Deflection, that's 84% and 75% respectively. Here, Arcane Deflection provides the same benefit in each case, but the gap between Resilient and War Caster was smaller in the first place.

From a practical standpoint, if you are using your Arcane Deflection to maintain your Concentration, you already casted a good spell that worths this investment. So Arcane Deflection really pays out and it’s an amazing feature for any Wizard.

Just use Mind Sliver and be happy. This cantrip is perfect for this situation.

I prefer resilient personally, because it applies to more then just concentration checks. Clearly both would be better if you can mange it. War caster also gives a good amount of other things that resilient doesn't but my preference has always been for guaranties mitigation of risks in a game of chance instead of an additional chance. The math is a bit too complicated for my post-work brain to deal with, but more often then not situations that I lose concentration on a spell are because the damage I took was enough to render either option irrelevant. 

I think it comes down to preference. I would recommend resilient, especially if you already have a way to give yourself advantage on the concentration check.