Having owned both cars.... I can honestly say, just like everyone else... that these two cars are completely different. I had a Procharged 97 GT , 4.10, full exhaust ( long tubes, pro-chamber, dumps ) and i could barely make a turn or keep my nitto drags from spinning. It was fun as hell as scary too, thinking back.
The Boxster isn't going to snap your head back with acceleration, but a high speed turn is just as exhilarating , so here is where they differ. Plus, matinence and parts come much cheaper in the AM world.