Nick's BJJ is nowhere near as good as Penn's in MMA and GSP wouldn't be afraid to take him down like he was with Shields. I don't think this really counts as a "superfight."
The only guy who's gonna beat GSP at WW is Shields, that's if he doesn't move to MW.
Well let us remember, BJ is obsessed wth not losing guard, whereas Diaz will use a lot more aggressive BJJ off his back.
But the more I think about it, GSP top control will be enough to dominate Diaz on the ground and I can't see GSP standing with him for long at all. Unfortunately Diaz doesn't have big one punch power and his pressure style on the feet isn't going to get a chance to pay off against GSP who will just take him down at will.
The more I think of it, the easier I see GSP taking this decision despite how far Diaz has developed and come over the last few years.