There's no possible way to say who's better, but you could say the Celtics play different game with Stiemsma.
Defensively, Perkins was the paint presence. When you wanted to cut, you'd run into Perkins - if you wanted out, Garnett was all over you. He didn't block that many shots but any 7-footer is naturally bound to block some easy ones. GS, however, isn't more of a presence in the paint than a robot hand guarding the rim, strapped onto the shot clock. Refs call basically any contact on Stiemsma a foul which is quite reasonable, but they are starting to give him some slack (2 charges that refs easily called fouls few weeks back). He's had to play weak and cave in the paint because he's always had the foul trouble. When refs start respecting him more - expect to see a lot tougher D from him. As for his shot blocking: pay attention to his attempts to block, that's maybe the most talent for blocking shots anyone currently has in the NBA. Incredible.
Offensively there's not much to say: We've become a shooting team, Perkins never shot the ball, he'd (we all remember Tommy saying this as I noticed) take the ball down to his ankles and then try dunking even though he's got 3 players on his back (literally). Stiemsma is fast, agile, he finishes below the rim and shoots well as of late. Remember he used to miss a lot of jumpers early in the season because he lacked confidence. Now he hits a lot. He fights for rebounds just like Perk did, except he does it with a lot more agility than Perk. He's a lot faster but doesn't box out as well as Perk did, enabling Pierce and KG to crash the boards.
We became a no-rebounding, shooting team once Perk left. Stiemsma fits into that perfectly. He's the first player up the floor after Rondo, unlike Perk who crosses the half-court line at 14 seconds left for the attack. However, we look as soft defensively as EVER. We're still really tough, but no team attacks us with fear any more.
My heart is with Mr Kendrick The Scowl Perkins, but Stiemsma, in his rookie year, already bought my desire to have him for a long time because he will always be underrated.