Those comparisons are ridiculous. Nick is going to be the most well rounded aggressive fighter GSP has faced in a very long time. He has the best chance of winning due to his willingness to put himself in danger for the reward.
I'm not saying he'll win but he's definitely the most dangerous person GSP has faced in awhile. We might even get to see GSP finish someone since Nick isn't afraid to put himself out there like that.
Shields has nowhere near the level of aggression that Nick does. Shields also has nowhere and I mean nowhere near the cardio level Nick has. They're completely different fighters. Shields might have more technical ability in the gym but Diaz has more tools to put those abilities to use in a fight.
I still say Diaz's cardio training isn't that applicable in MMA or fighting in general. He trains for triathlons and other long distance situations. As far as fighting goes.. that's sort of just a waste of time. In fighting you need explosive energy and fast recovery...