The American ones get like exponentially better after that point, actually. Like, from the sexual harrassment training episode on.
Anyway. As far as the cast goes, David Brent and Michael Scott are practically entirely different characters to me at this point who just share a common archetype. They definitely made Michael much more sympathetic, but I think they did it effectively. I like David Brent more, though, if forced to choose.
Jim/Tim is another one where the characters are mostly the same in role more than character. Tim is far superior in the role that it's supposed to be, if that makes any sense, so I prefer him to Jim, but John Krasinski plays the straight man role brilliantly either way.
For Dawn versus Pam? Pam by like ten thousand miles. She just... I don't know, does more with the role, it feels like. She's just sort of funnier and she does the serious stuff better.
Gareth vs. Dwight, oh god. Both brilliant characters, but I think I still enjoy Gareth more. Like, I think Dwight is funnier in a way, but Gareth is the better character. Plus Mackenzie Cook is a better actor than Rainn Wilson, in my opinion.
I honestly think that the American version has the superior supporting cast than the British version, though. Mostly because it has a real supporting cast. Like, Kevin is given more to do in the American version than Keith got to do in the British, and there's nobody in the original as far as minor characters goes who could hope to even touch the brilliance that is Creed. Plus Angela, Stanley, and Toby are all fucking hysterical as well. Chris Finch owns Todd Packer, though.