GSP's hand speed and foot speed will be too much for Nick in the stand up. Think GSP vs Fitch. And on the ground I think GSP will pound Nick's head through the canvas the same way he did Penn. There is a reason GSP asked for this match up. How does Nick make it out of this fight?
First, suggesting that GSP-Fitch is in anyway relevant to GSP-Diaz, in terms of standup, is ridiculous, IMHO. Diaz is well ahead of Fitch in both footwork and boxing.
Second, Diaz has fought (and beaten) guys like Daley and Noons, who arguably have *at least* comparable boxing and hand speed to GSP (I would suggest they are both better/faster in pure standup).
As for what happened to BJ when he fought GSP, I fail to see how that is relevant to Diaz at all. They are two very different fighters, and BJ has no business fighting at WW anymore. Diaz also beat up Penn, so doesn't that at least suggest that GSP won't tool him the same way? If you at all buy into the greasegate thing that Penn alleged, that will also not happen, given the changes implemented for cornermen.
I'm certainly not saying Diaz will win, but I *highly* doubt that he will be stopped by GSP, who will not risk position on the ground against someone with Nick's level of BJJ, which is almost certainly equal to (if not higher) than that of GSP.
Diaz will also not gas the way Penn does in almost every WW fight that he has lost/drawn.
So...I'd flip this back on you and ask, how does GSP stop Diaz? He has stopped two lightweights in Penn and Serra, and a past-his-prime Hughes, in his last 10 fights, and Diaz hasn't been stopped in OVER TEN YEARS outside of a cut TKO loss to Noons over five years ago.
GSP by decision, in all likelihood...but I would bet heavily against a stoppage win.
edit: add Zaromskis to the list of capable strikers Diaz handled...