The difference between the older ZX-7 and the newer ZX-6 isn’t going to be dramatic. A 600cc sportbike is a beast compared to an equivalent non-sport bike. Period. Depending on engine response, a 600cc can power-wheelie, but not as easily as you might think. Lifting the front on the 600s I’ve ridden requires that you abruptly slam on full throttle in the powerband of a low gear, usually first. Accidental power-wheelies are only an issue if you’re not patient in learning to be smooth on the throttle. Be sure to spend lots of time getting acquainted with your bike in closed areas (read: parking lots, etc.) before getting to adventurous on the road.
In a short, the ZX-6 is basically just a lighter, slightly less powerful ZX-7, but with two years’ worth of new research technology. Not much different.