Well I`m a bit bias cosI`ve got a K1 GSX-R600 and love it!
I had the same dilema you have now when I bought mine. This is what made me stick with the Gixxer
1, The ZX6 is VERY tarck focused and not very comfy on the road. The extra 36cc (636cc) hike it over into the higher tax bracket for road fund licence as well plus I recon it would hike teh insurance a little as well.
2, The R6 is VERY skinny compared to the Gixxer It felt too narrow for me. I didn`t like the clocks on em either.
3, The GSXR. It`s Phat, wide, when cranked over and hanging of you can hug the tank with your arm. The induction roar is awsome. The brakes on the erlier ones pre radial (<04 I think) aren`t as good as they could be. they are far from rubbish, just that the competition`s are better. The GSXR makes a fantastic track tool out of the box )I`ve done Brands Indy 3 times and Silverstone once) And with some inexpensive tweeking can go even better
Well that`s my pennies worth any way
Hope thats of some help