i think i have to go with GSP. BJ Penn is beginning the downhill of his career in my opinion. back when he was fighting lightweight, his fights didnt last longer than the first round usually, and even when he moved up, he proved he was tough to beat. but since his first bout with GSP, it seems to me that hes not the same guy anymore, he lost his down to earth-ness and i think thats why hes not at peak performance. I think if you put BJ in a real good camp on the mainland where he takes some licks, he could easily take the belt from GSP and hold both belts for a long time. but because hes so good already, no one can give him the kick in the pants he needs to step down and toughen up... except GSP, maybe. again, all my opinion
GSP on the other hand is at the peak of his game. i dont think hes ever been in better shape than now. he keeps improving. id really like to see him land that huge spinning sidekick one day. when he begged for the title shot after he beat Matt Serra, that was cool. down to earth, improving his game solid champion.