Nalby is hot right now and has one of the best returns in the game so in theory he should have the advantage. However the court couldn't suit Isner's game better (home court advantage included), Isner can get more kick on his serve than any other guy out there which will hurt someone "short" like Nalby no matter how good his ROS is, and Isner is the toughest guy mentally out there while Nalby is famous for the opposite.
Isner in 3: 6-4 3-6 7-6(4)
Whether he's hot or not has no bearing on the outcome, Nalbandian is ten times the player Isner is. If Isner serves well, he could force a tiebreak. If he serves poorly, he's losing 6-1 6-0.