Which Brave Ike is better? Atk+ /-Res or Def+ / -HP

i think that +def is sligtly better than +atk, if you increase his atk his first hit will deal more dmg and when you proc aether in the second hit the enemy will have less hp and you will heal less from aether.

Mine is +Atk -Res and he works just fine and I think that one would be better since I wouldn't want to reduce his HP even though his Def is a boon.

I'm kind of new to B.Ike, but from what I can tell, -res is only bad against dragons, especially if he gets doubled, and one-shot mages. You're kind of gaining mileage with -res against fast mages because his res is almost irrelevant on the second hit.

+atk is indeed better for Aether.

Generally, HP banes are good if you don't need your HP for panic ploy or supplying infantry pulse and if you're likely to get doubled. With B.Ike it's a bit different due to Urvan's deflect everything; -HP is preferred if he gets doubled by a ranged unit and not preferred if he gets doubled by a melee unit. So +def -HP is a good IV for melee tanking, and +atk -res is a good IV for ranged tanking.

As to which you go with, I think it depends on how you want to use him, because he's probably excellent either way as long as you build accordingly.

I got like four different Brave Ikes including one with your iv's and +Atk -HP my experience with his base kit and QR seal, the best one was +Def -HP, surviving more matches and getting more HP via Aether, so I recommend you keep that one