UK, hands down. I like the US version, but it doesn't have the subtlety of the original. A lot of the humour in the original stemmed from how monotonous it was, and how mundane their lives are. There are fewer moments like that in the American version, and it seems a lot more like a traditional sitcom.
As funny as Dwight is, he's a bit over-the-top. Michael can be too, at times.
I think Jim and Pam are both perfect though. And Jenna Fischer is cute as a bug's ear.