orcus - "when skill is equal or close, the winners always gonna be the bigger stronger man, its why we have weight classes."
It sounds like you're saying GSP is not a better fighter, he's just bigger, and so pretty much avoiding a straight answer to Ausgepicht's question.
At what aspect of MMA is BJ better than GSP?
How much to remove from consideration the weight advantage when comparing skills is a tough endeavor. But I would still say BJ's BJJ is better than GSP's, despite being passed numerous times in the last fight.
Weight/strength, in my opinion, has a pretty substantial effect on guard passing, especially for GSP's bread and butter "no hand" pass. While there is no doubt GSP has technically perfected that pass, the particular pass relies a lot on the ability to have strong head and arm control. If I had to quantify it, success at that particular pass would consist of 60% skill, and 40% weight/strength.
Thus, GSP's guard passing clinic, in my opinion, does not necessarily demonstrate that GSP's BJJ skills are better than BJ's, independent of weight.
Moving on to other aspects of BJJ, I think BJ has demonstrated a more diverse BJJ game in MMA.
GSP has demonstrated a strong top kimura. From my experience, jiu jitsu players who habitually go for bent arm-locks from the top are usually stronger, more athletic guys. I think the reason is because strength is a larger factor in finishing a kimura than with other subs (compare, for instance, the strength required to control someone and finish a kimura from the top vs. a RNC). Also, his kimura opens up transitions for him to go for straight armbars (Hughes III and Mayhem (?) )
On the other hand, BJ has shown good armbar-triangle transitions against Pulver, a reverse triangle against Hughes, gogoplata attempt against GSP, and has shown a high percentage of finishing the RNC (Hughes I, Gomi, Stevenson, Pulver II). Needless to say, BJ gets the back a lot more often as well.
Finally, BJ's top game exhibits more technical passing (see, e.g., Pulver I). While GSP's head and arm pass is very effective, BJ seems to have more of an arsenal.
The foregoing are my reasons for concluding that BJ's BJJ game is still better than GSP's, if weight wasn't a factor. By no means am I a BJJ guru, so please feel free to jump in and disagree.
I can't speak to striking, b/c I have no experience there, although GSP did show better striking than BJ in the second fight. And there is no question that GSP's wrestling is better, if not the best in the sport.