Wallace would probably be the smartest move. Perk would definitely start, I'm sure KG would much rather have him by his side on D. A PF's ability to spread to floor is very valuable yet underrated. I know Scal doesn't score much himself, but i honestly notice an increase in other people scoring when he's standing behind the arc with his hands up. The Magic seemed to have success with four 3-point shooters on the floor at all times, and they don't have drivers like Pierce and Rondo.
Also, Perk is essential to defend guys like Howard, but i wouldn't mind seeing Wallace play excessive center minutes for Perk. That would give Garnett the opportunity to play a little closer to the basket (you know Rondo feeds those ally-oops perfectly).
I would rather see Wallace in the game for either of them than Big Baby to be honest (as long as can fit in nicely). And if we need some reserve muscle that's what we got Powe for. (HOPEFULLY)