If you can test ride both, do so... After that, it should be easy to decide, but here are some things to consider, no matter what. With regards to the FZ6, it is similar to the older, FZR600, nearly the same engine: smooth, very quick acceleration; nice, comfortable suspension -- quite nice actually; if it is a lean-forward rider position, walk away, unless you plan on short trips only.
My son had a '94(?) FZR600. Really a very nice bike, aside from the forward-leaning position, which I could only tolerate for less than 30 minutes at a time. Other than that, it was nice and comfy, but with more acceleration than I needed/wanted. It ran smoothly, powerfully, and comfortably, but
it only had three different tires that would fit it -- PERIOD! Check on tire availability to make sure you have several makes and models to choose from... If it's an odd-ball size, walk away. I like Yamaha bikes, but they're sometimes a little too far afield in some areas. Check out the rear view mirrors, and how easy(?) it is to look behind you, when gripping the handlebars: my son was speeding, and never saw the LEO's for several miles, they called it in as a high-speed chase, threw him on the ground, cuffed him, and put him in the squad car, in an obvious state of anger towards him. He never sassed them, he acted very politely, and explained he could not see them in his mirrors. One of the LEO's sided with him, saying he had one of these bikes before, and that the mirrors were worthless, and looking back was very difficult (the arresting LEO threw a leg over, and was finally convinced). They finally let him go, with just a ticket. This bike's mirrors look like they are on long enough stalks, but be sure to check them out carefully. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII