Muller has got so much talent and when he plays his best game is such a joy to watch, one of his USO run [2008 I guess] was phenomenal , and he has beaten big names in Slams like Rafa in Wimbledon, but unfortunately we just don't know which Muller will turn up.
Isner is very consistent and has improved several aspects of his game including court coverage, he can play from baseline and win points.
I want Muller to win , but I guess Big Johnny Bravo will do it in 4 tight sets.