I think it really comes down to which one is in best shape, has the right mileage, and history. As others have noted, '97 had some minor improvements (forged wheels, better weatherstripping, slightly better epoxy work (that's my opinion)). The '96 was first, though, so that might be worth something - but not much in my opinion. Some people like the stripes going through the license plate frame, some don't care.
One thing to consider is that there were actually fewer B/W '97 GTSs made than '96s since they also made a red one in '97. It makes the '97 B/W slightly rarer (fewer than 1000 were made compared to around 1200 for the '96s).