To me they're two very different things and it's difficult to say which one I prefer more. With the SP games you get a game made around you that you can play on your own time, and I love a good SP game. But with an MMORPG you get
Massive Men Online Role-Playing Girls a massive community with tonnes of other players who want to have fun and kill dozens of rats who have money for reasons best left unexplained.
I'm going to say both, but nothing is as good as playing an online game with a group of good friends.
Let's kill ourselves.