Between two in the pocket fighters you usually have to favor the guy with more firepower. I don't see enough things stylewise for Diaz to make a difference in his favor. He'd slip some punches and make it entertaining but would get taken out mid to late.
not sure how good rios is yet. based on what i've seen, i'm still not all that impressed. he's fun to watch though. i think it's a pick'em fight.
Diaz doesn't have the power to keep Rios off him. Would be a good action fight but I see Rios stopping Juan. Juan would give a good account of himself and would get his shots in but I don't see him hurting Rios enough to slow him down.