Lacking in power doesn't mean lacking in fun. Buy that 250 and ride the wheels off it. If you get hooked by the track experience, you can sell the Ninja and move up to an older but well set up 600. You'll never outride that.
That's my thoughts. Ride something small like I stole it instead of something too fast for my own good learning.
I'm mostly a dirt rider and some of my most fun I've had was trying to run down my friend on her WR250F with the TTR125 I owned at the time. She could beat me in the straights, but if you have no intention of ever touching the brakes you can catch the big bikes in the corners :laughing