I do both, and I love them both.
They are two totally different sports, and its great fun to be able to switch between them in the same afternoon.
In Safety Bay it definately helps to know both sports, as you know who wants to go where, so it causes less agro. In my opinion, its the lack of knowledge about the other sport that causes all the agro.
Windsurfing in the waves is definately way way better, but on flat water, theres nothing better than jumping 5-10m in the air, whenever you want. But i do love freestyle windsurfing as well. I think kitesurfers going to windsurfing would give up because its too hard and they'd be used to learning things quickly!!
Anyone know of anyone who's gone from kitesurfing to windsurfing, I know one person...rare case i think