If you only use it for commuting, I'd probably stick with the 600 and spend the difference on some upgrades. You probably won't go any faster on the 1000, and it will cost a bit more to run.
That being said, if you want to change bikes because you've had enough of yours, go for a thou. It's getting hard to find really good 600s now, whereas there are a lot of good 1000s about, people tend to look after bigger bikes better, 600s become hacks!
I kept my 51 plate 600 for 6 1/2 years, the longest I've kept a bike in 40 years of riding, px'd it for a new Honda CB1000R, but ended up coming back to a 54 plate 1000, about 14 months ago, because they're just such great do-it-all bikes!