David Tennant was always going to be a very tough act to follow but I think Matt Smith has done brilliantly so far. As for who's better, I really don't know. I prefer Matt Smith's character in a way, he comes across as arrogant at times which is something that never happened with David and I think it fits the super-intelligent alien persona much better. So I prefer Matt Smith's character, but I don't really have a preference when between the actors themselves.
Btw, does anyone know when the show is coming back?