Well, I just test drove an '01 GTP with 28K and every option and a '97 GS with 39K and most options (no spoiler, Bose system or moon roof) and the power seems the same, but the GS feels more, um, uh, Buick-like... you know, softer suspension. The GS had less road noise than the GTP, but that seemed to be because you don't hear the exhaust note as much on the Buick. My personal preference goes to the GTP in the looks dept, both inside and out. It just looks sportier. I think the GS, especially equipped without the spoiler, makes for a great sleeper. Besides, who needs rear deck spoliers/wings on front wheel drives anyway? More downforce on the rear wheels is the LAST thing you'd want.
That's my nickel's worth, anyway.
(inflation, you know)
PS - that said, I ended up buying the GS because my wife liked it better and it was a lot cheaper than the GTP due to it's age.