Personally, I like riding the R6...but that is only a personal opinion and I know you didn't list it but thats my $.02, lol, just because I like the feel of it better. But out of what you listed my next choice would be the ZX-6R. They make great power and are a pretty light bike and usually (at least in my area) they are the least expensive. It seems like they are always advertising some sort of promotion for them. For me when I sit on a new CBR they sit way to high (I'm short-5'8") and the GSXR, for some reason I feel top-heavy. Hopefully in your area your dealers will let you take a quick test ride on them so you can feel which one is best for you. A lot of dealers won't but a few around my area will, so it wont hurt to ask.
When you get comfortable on the 600's see if you can find someone willing to let you ride the ZX-10R, WoW what a ride! Its not the fastest out there, but I am very comfortable on that bike going through the curves in our area. That is on my list of purchases this summer!