I think the main difference between the U.S. and English versions is that they made Michael Scott more likeable because he would have glimpses of being a decent guy that the other characters would respond to, and deep down for the most part the other characters actually liked him, whereas David Brent was more of a schmuck that the other characters despised the whole way through the show. The only time he had a redeeming moment with the rest of the cast is when he gets that laugh at the reunion party. I found David Brent to be a more interesting character in that regard, but Michael is not far behind. I haven't seen the Christmas specials, so I can't comment on them.
Also, the side characters in the U.S. version get more attention and bigger roles, which makes it seem like a fuller show. If I had to say which I like better, I'd say the U.S. version, but English one is still good.
The U.S. show gets weaker after Michael leaves, but it never strays into being a bad show. Like most U.S. television, it went on longer than it probably should.