lupa rey or violetta?

Lupas brain gives him 200 bonus health.
Reys brain gives him bonus attack.
Violettas brain gives him bonus move speed.

200 health is a massive bonus to health. I dont know how much the attack bonus is but I would say that the move speed is the worst of the three.

also the brain you give him will affect the ending of the game, rex will go back to wherever you got the brain from

Hi Caligula,
I went to that gas station to kill the woman, but her dogs don't attack me...
(I have the Animal Friend perk.)
So I went to Old Lady Gibson.
Man, it was priceless - I laughed hard! A can't-miss moment in the game.
Now old Rey will never die...
You should have seen, once I took him back by the Gibson scrapyard - the other dogs were sniffin' him out, it was kewl.
The only reason you'd want to get him more health is if you're on hardcoe mode... (which I am not).
Regards