I love both of them, I mean I can play "World of Warcraft" as well as any of the RPGs such as the "Tales of...," "Fire Emblem" and etc. Although my favourite RPG would be "Tales of Symphonia 1".
Hm, hard one because currently I'm only playing World of Warcraft..
Remember EverQuest?
That was good back in the day, but then Warcraft took over.
While I love both, I prefer multiplayer games, just for the fact that NPC dialogue gets tiresome. I enjoy the feeling of knowing there are others about, even if I don't interact with them much.
I would have to say that I prefer single-player RPGs because the only MMORPG that I've ever liked is Ultima Online.