The major difference is that the British ones actually do a good job of painting the Mac as someone you wouldn't mind being identified with. The American Mac comes off as smug and condescending while his cousin from across The Pond seems a lot more sympathetic. He doesn't have the superior glint in his eye nor does he seem to take any enjoyment at PC's series of blunders.
The American ads don't quite work for the same reason there are so few American sitcoms that are genuinely funny. Apple USA is terrified that you won't get the point and so try too hard with their message. If they just dialed it back a bit and gave the audience some credit the spots would actually work.