i have a zr-7 and my friend has a 93 zx-6 (not the r). i've ridden both for many miles. the other day we went riding and i had the chance to ride them both back to back.
overall, the ninja feels more solid, more planted while riding. road feel is a bit better. the ninja definitely has more top end. the zr-7 has more leg room from peg to seat. it's also more buzzy.
another difference you might notice: cost. not just payments, think about insurance. it may be significantly higher on the ninja for full coverage.
the ninja is a race bred machine with all the pro's and con's that entails. ask yourself this: are you happy with the zr-7? does it do everything you want it to do? that's the real test. make your own decision, don't let others dictate your choice of ride.
scott