Since one is an MMO and the other a single player game which has an actual "ENDING," my vote is Skyrim; especially since I've already been playing it.
I've put in well over 80 hours on Skyrim and had to stop for work. With an MMO, I'm pretty sure I'd have a hard time quitting, especially if my friends are also playing.
If you don't have much free time then I'd say Skyrim. There's an end, but you're welcome to continue doing additional quests for as long as you want (I imagine they would eventually get quite boring though!).