I prefer SP.
-Most people are idiots. In SP mode, you don't have to deal with people, only scripted NPC's and AI-enabled characters.
-It's easier to have an interesting story in a SP game. MP games just say "kill those guys". Why? Who knows, but if you succeed, you win. Some SP games are like that, but there are SP games with great storytelling where you really feel that you have a purpose in the game.
-You can play for 30 minutes a day or 6 hours. Just save when you're done and when you resume later, everything will be exactly as you left it.
-SP games are more balanced - the levels are adapted to your skills as a player and to the abilities and equipment of your character. SP games can be challenging, but they're made so that you can "win" eventually. In MP games, that's nearly impossible because of cheating, exploits or simply because some people have way too much time on their hands. If you're working or studying or just happen to have a real life outside of gaming, it's nearly impossible to win.