I've gone to both this year.. this is how I look at it..
Ghostrider > Grizzly
Drop Zone > Supreme Scream
Invertigo > Boomerang
Top Gun > Silver Bullet
Xcelerator > Everything
They were both nice parks though and you should have fun at either. I personally thought KGA's coasters were less thrilling.
I went in the morning to buy donuts and when I got home my dad said "Holden, Six Flags just called... something went wrong with the power lastnight and the whole park is closed today!" And it was early (and I was tired) .. I believed him..