Never got into SP, but I think it's a bit like comparing dancing to playing tennis. Which requires more skill to be great? Well, I don't know, but they are very different activities. SP is more like dancing, as there are no hard limits placed by time or opponent, and your only objective is to make something that looks cool. With tennis or MP you have to worry about the ball or opponent's foot, in addition to time limits. You can try to look good, but your creative freedom is limited by your opponent and time constraints.