Advantage or extra damage

I was wondering what would be better to have advantage on an attack roll or to have an extra die of damage.

My character build has two options I could take advantage on all attack rolls casting GFB or adding an extra 1d8 on top of the 1d8 from GFB. The magic weapon at present does 1d10 +4 + 2d6, which would be the best option?

So it would be advantage 1d10 +4 + 2d6 + 1d8 or normal 1d10 +4 +2d6 + 2d8.

Just to be a bit clearer I have the choice of taking Magic Initiate feat, getting GFB, and getting a level that would give me an extra 1d8 on the attack  or taking a level of sorcerer instead and getting GFB and using Innate Sorcery for advantage

Thanks

So, I'm not going to get too much into the math here, but you need to consider what your target die number is for an attack (which will also depend on the AC of your average target). In the best-case scenario, advantage adds roughly +25% chance at a target roll of 11 and tapers off as you move the target number in either direction (low target number means you didn't really need advantage in the first place, and high target number means you are still not likely to hit it). So at the best-case scenario, you are looking at +5.25 damage on average (25% of the 21 damage, on average, you do on a hit). Adding another 1d8 is +4.5 damage on average. Seeing as +4.5 is about a +21% increase, any attack where the target number is between 7 and 15 would give advantage the better payout.

Then you also want to consider that advantage gives you double the chance to crit (+4.75% if adding percentages)... which is double damage (you'll want to consider if all of the dice get doubled here, or just some).

In conclusion, unless your bonus to hit is so high that you are basically going to hit no matter what (or miss on anything but the luckiest of rolls), then advantage is the way to go.

PS: I guess you also want to consider if you have other ways of getting advantage on a target and how easy that is to achieve, seeing as advantage doesn't stack.

Thanks

My build is starting at level 6. I have a Vicious club, I can cast shillelagh on it and use GFB, I attack with a +7. But I have choice to make either a level 5 druid  and level 1 sorcerer and get advantage on GFB attacks or a level 6 druid take Magic initiate and get GFB but get an extra 1d8 from wildfire spirit.

I didnt think about the about possibility of critical hits or of mitigating critical misses.

Something else to consider is that with only one level of Sorcerer, you can only use Innate Sorcery twice per Long Rest. Wildfire Spirit lasts a lot longer than Innate Sorcerery (1 hour vs. 1 minute) and it uses your Wild Shape charges, which at Druid level 6 you'll have three of per Long Rest, with one coming back on every Short Rest.

So with these specific abilities, the extra damage from Wildfire Spirit is likely to be available a lot more than the advantage from Innate Sorcery.

Note that Innate Sorcery & Shillelagh are both a Bonus Action so you won't be able to use both on one turn. So you aren't getting Adv until round 2, and then only for 2 combats per day. Wildfire spirit costs a whole action to summon but it does last for 1 hour so you have a high chance of being able to summon it before combat starts. 

magic weapon at present does 1d10 +4 + 2d6

You should double check that. A Vicious weapon deals a little extra damage on a critical hit, it certainly doesn't deal 2d6 extra damage on every attack. Are you using some other spell to further boost your damage? 

But I have choice to make either a level 5 druid  and level 1 sorcerer and get advantage on GFB attacks or a level 6 druid take Magic initiate and get GFB but get an extra 1d8 from wildfire spirit.

I'd highly recommend taking Magic Initiate as your background feat as a Druid, pick up GFB but also for Shield. So many druid spells are concentration if you are going into melee with a club you really want as much protection for your concentration as possible. Plus as a druid many of your best spells are those that grant advantage on enemies so you shouldn't be using a Sorcerer dip to get advantage - throw up Faerie Fire or Entangle to get your advantage. 

The damage increase from advantage is equal to your miss chance without it -- i.e. if you have a 30% miss chance without advantage, it's an effective 30% increase in damage to have advantage. As your current damage is 16.5 and 4.5 damage is 27% of that, the extra damage will be better against low AC.

The other question is whether GFB even benefits from either effect (this is a problem with all the blade cantrips).

I recommend using the RPGBot DPR Calculator to play through scenarios. It doesn't handle Vex, per se, but it is good in general.

For an on-CR encounter, you would have expect to have a 60% hit chance and a 5% crit chance (65% total hit chance). If you have a below or above average bonus, you would increase or decrease the Min. to Hit value accordingly. (At level 6, it sounds like you have an average chance, so you don't need to make any changes). You can change the accuracy to Disadvantage, Advantage, Normal, or Elven Accuracy.

I think all dice damage is multiplied on a crit, so advantage 1d10 +4 + 2d6 + 1d8 would be a DPR of 20.09 and normal 1d10 +4 +2d6 + 2d8 would be a DPR of 17.65.

I hope that helps.

Quote from Agilemind >>

A Vicious weapon deals a little extra damage on a critical hit, it certainly doesn't deal 2d6 extra damage on every attack.

The 5.5e version of Vicious Weapon does indeed do 2d6 extra damage on every hit against a creature.

I think after all your advice and  RPGBot DPR Calculator , thanks SmiteMakesRight_3_5,, Im going level 1 sorcerer and level 5 druid, As for Feats Tough for origin and warcaster, for conc advantage.

I checked and dnd 5.5e vicious weapons do 2d6 on every hit.  At the moment we rarely do more than 2 combats per day.

My plan on starting each combat is Innate sorcery and firebolt, then if they get in close Shillelagh and GFB, with advantage from Innate sorcery. If I have a turn to prepare wildfire spirit as an action and Barkskin on the spirit as a bonus action

Yeah, vicious weapons are a bit overtuned for their rarity. 2d6 per hit is about scaled right for Very Rare, Rare should be more like 1d8 (or maybe double base weapon damage dice, damage bonuses are less useful when damage is already high).

But then Im only getting 1 attack, while others are getting 2 to 3 with a damage mods :)