I myself must say SP. The MP for me is good I consider myself avg in MP.
I am learning MP is not so much the game play as it is the interaction with others online.
I enjoy doing missions together and helping people get started, as will as PvP in something sanctioned by the Server. Where you get a reward for being #1.
In addition, you can always go for that big kill list over at the Global Stats web site. More to do in MP then in SP that is for sure.
