I could get into a very long post on the differences, but I think the primary differences between the books are:

1) the worldbuilding. JK wrote a world that people still want to wallow and play in, whereas Meyer's series is based in the Real World (and we won't talk about how much of the Pac NW she got wrong.)

2) the sources of conflict. JK's is a Good vs. Evil story with Harry and his comrades battling Voldemort and the end of the world, whereas Meyer is relying a romantic conflict (with religious overtones, control-issues, and bonus stalking!)