What Robert said. It's like saying "I'm thinking about buying a Ford Mustang GT or a Ford F350 Super Duty, which is better?"
Not a slam, just two very different bikes, although they may look similiar (sportbike looks).
ZX12r, porky, not good in twisties (compared to gsxr750), BUT more comfortable, more power, quicker in a straightline, higher top speed. More of a sport tourer geared sportbike.
GSX-R 750, light, flickable in the twisties, better brakes, BUT less comfortable, less power but not much slower (limited by front wheel staying down on both, moreso on ZX12r). Pure bred sportbike, meant to go fast while leaned over, and do everything else great.