Depends how much you're gonna be on it, and in what you mainly use it for i s'pose.
I nearly bought a 1000 a couple of years ago but as it's mainly a work commuter for me i stayed with the 600.
I'm in traffic a lot, then it sits outside my work all day so to me it seemed pointless.
The 600 is a cheaper all rounder for parts, insurance, fuel, lighter in weight, etc, but still has more than enough poke to have fun with whenever you feel like it.
That's my circumstances though, if i did more mileage, carried passengers more, etc, then i probably would consider the 1000 straight away.