I wouldn't go so far as to say that you're not ready for a 600, at least not yet. What's your experience level on two wheels? How many months/years? How many miles have you done? Track days? Years on dirt?
Another one that a lot of people lie to themselves about... what's your maturity level? Bigger bike can get you into a lot more trouble quickly, if you're not careful/mature about when you open the taps a little bit. But, at the end of the day, the bike will only go as fast as you make it.
Finally, as a side note, depending on your age/riding experience, you might want to get some insurance quotes first. Beginner-ish riders on supersports are not cheap to insure... make sure you're ready to pay the premiums before you buy.
I had my Ninja 250 for 2 years and put about 3k miles on it and was no longer enjoying myself on the bike as it I felt it was time to upgrade. I sold my bike not too long ago and had to focus on other personal finances. Now that everything is worked out, I am looking to purchase another bike but I don't want a 250 or 300 and have always loved the cbr600rr or r6. I am 26 years old and I don't think I would have an issue with insurance costs.
