The ZX7 is 50lb heavier and 20bhp down on power...compared to the zx9 C1 model and newer... It is more uncomfotable for both rider and pillion and has a shorter tank range...115 miles, compared to 140miles for the zx9, (depending on how you ride obviously).
I've ridden both on track and the ZX7 is not the track demon many owners would have you believe, especially as standard when compared to more modern tackle..It's slow to turn as you simply can not get away from the weight..
The ZX7 however does respond well to engine tuning, and raising the rear ride hieght does help the bike turn.. Also it does have a nice front end feel going into bends and once laid over is confidence inspiring...It's also very stable, which is aided by the relatively modest power..
The ZX9 is aslo a little ponderous on track due to its size and slightly more flappy, due it's lighter weight and greater power, but could still be regarded as a stable bike...Overall. as standard., I would say the ZX9 is the better track bike, mainly due to the extra power and ligter weight..many ZX7 owners disagree, but they will own a modified ZX7 or have never ridden ZX9..
You could argue the ZX7 looks better and is quick enough for the road..
The ZX9 is clearly a better road bike and arguably a better track bike.
Either way, you buy a ZX7 with you heart and a ZX9 with your head..
I've owned three ZX9's and two ZX7's..so this is my unbiased opinion..