I've always been quite fond of Maitland's drumming, particularly during the big build-up towards the end of "Buying New Soul" with regards to the "Lightbulb Sun" era. While I don't think there's much question that Harrison is the more technically proficient drummer, I've always found Maitland's drumming to be very musical - almost melodic, e.g., Waiting Phase 2 intro, Russia on Ice ending..., and a major component of SW's earlier years (including his No-Man contributions). Also, he always brought some serious energy to the live shows (Coma Divine, for example).
Now I'm not saying that Harrison isn't melodic, or energetic, of course; they both just brought different things to PT. I have a hard time imagining In Absentia, or anything after, with any other drummer but GH. At the same time, when I hear GH playing pre-In Absentia material live, often times I prefer Maitland's original approach.
But, hey, that's just my opinion. The Maitland vs. Harrison discussion has been going on for many years. I think Signify through Deadwing are some of the finest albums released in the past 20 years, and wouldn't change a thing about them.