Any of them are going to be much more uncomfortable than your current setup for the street, with the current R6 and Zx-6r being very torturous on the wrists and back. I know a bit about the Yamahas as I have rode all of these generations.
01/02 R6 - I owned this bike. Great engine for the timeperiod. Seating position was better than current SS bikes. Carbed. Brakes were only meh...
03/04 R6 - My buddy Brett owns an 04. Better step foreward. Fuel injected. A little bit more focused seating. Much better brakes.
05 R6 - My best friend Matt owned an 05. Bigger throttle bodies, inverted forks, and 70 size front. Stiffer feel into corners.
06+ - Track bike with lights. Not worth buying for the street.
My advice would be keep the YZF600R for the season and get some good miles under you. Of the three brands you mentioned, I would say the 2005 R6 is the best Yamaha option. The 05/06 Zx-6R 636 is the best Kawasaki option. And as Derf said, the CBR F4i is probably the best street option for Honda. 01/03 had the split seat, and 04/06 had the banana seat.