the DF is much the better body. no it is not "just the price" - AF is the main improvement, but you can hear the difference when the shutter fires.
The 645AF is a good backup body and can still use FILM backs. the DF cannot, I believe. Also the DF can use the vertical grip, etc.
I too was very happy with the upgrade, but would be happier still if they improve the DF a lot more... It is the weakest link in the Phase product range. Lenses are very good, backs are world leaders, but the camera body is mediocre, and belongs to the early 90's, not 2011.
Hopefully the Pentax645 has raised the bar a lot on them, and they will be forced to bring out a radically better body soon, with multi point AF, faster handling and single power source for camera + back.