GO WITH GIMP! ITS HOT.
I go with what I'm familiar with. I've had Paint Shop Pro for years, and am now with the X version. For awhile my laptop was dual-boot; XP SP2 and Ubuntu 10. Gimp was provided with Ubuntu. I played around with it but found it hard to work in. Maybe it was just getting to know another application, but I always found myself going into XP when I wanted to work with graphics. Now I've done away with Ubuntu completely. That's another story...
I might download the Windows version of Gimp and try it, but I don't do that much with graphics so am not sure it would be time well spent.