I think just about the only British comedy I've seen that I absolutely loved was Coupling. And yes, despite being American, I have seen a lot of British comedy (I lived in Israel for a while and got a bunch of British channels).
So I guess I prefer American comedy.
No wait, scratch that. Isn't this kind of a stupid question? Not all American comedy is the same, just as not all British comedy is the same. Each side has it's own fair share of bad comedy and good comedy.