The one BIG problem with taking the 250 onto a track is that since you WILL be going slower on the straights, virtually everyone will pass you. That is all fine and dandy if they wouldn't park in the turn. What you'll find is that you will be held up in some turns by the people that had just passed you. Then when you get back on the straight, you'll watch them walk away from you, then you'll be back to them at the next set of turns. *sigh*
Now that probably sounds like I'm being all competitive, egotistical and stuff, but when you are paying for a school where you need to practice on track what was just taught in the class and someone is holding you up in the turn where you are trying to work on your skills, it gets very frustrating.
On a positive note, it REALLY is a lot of fun to out drive big bikes (like Aprillia 1000's ) through turns on Ninja 250's.
I've ridden a 250 at a track (Carolina Motorsports Park in Kershaw, SC) after doing Level 1 on a Suzuki Bandit. There is MUCH you can learn on a 250 at the track, BUT, I'd recommend you carry the 600. It sounds like you have a decent head on your shoulders and are truly interested more in learning to ride smart rather than just blindly grabbing a handful and hoping for the best. Your CBR was BUILT for the track and as much as I like the little 250's (I've got 2 of them), they just don't do as well. Your learning experience will be just as good, if not better, on the 600. EITHER BIKE WILL DO FINE, but I think the 600 would be the better weapon of choice.
Good luck!