If for no other reason than the fact that when Hughes was coaching TUF this past season he said on camera he wants to retire, and has maybe 3 or 4 fights left in him.
Although i'd take GSP in this fight regardless, he's way more well-rounded. Hughes' standup is very poor no matter how much people say it's "much improved", he still gets his azz handed to him on the feet. And GSP has proven he can wrestle with Hughes.
I think GSP is a ton better standing up than Hughes. And I think that GSP can avoid Hughes takedowns which is big. Like Pit said "he's way more well-rounded".