The 94-97 had tails with yellow, while the 98-01 had the all-red. And they were the same size all years. It's pretty common for someone with a 94-97 to replace the taillights with 98-01.

94-97 was the original body style, while the 98-01 incorporated a more aggressive "wide-mouth" front bumper, relatively flush headlights (rather than the recessed ones of 94-97) and a revised rear bumper.

GSR power levels remained the same all years, while the 94-95 LS had 142 hp. This decreased to 140hp in 96' with the introduction of OBDII.

Different year GSR's came w/ different rims. 94-95 had a traditional 5-spoke style, the 96's had six-spoke rims that most people don't like. The 99-01 had a "chromed" six-spoke "blade style" rim. I think the 97-98 had a multi-spoke design, but I'm not sure.