B-12 was the first thing I ever pinned and I've done it both ways. To be honest, I couldn't tell a single ounce of difference either way.
I have always said that B-12 was one of the most important supps I take. There's no way I could eat as much as I do without it. Some people say it does nothing for their appetite, but it does for me. I do all of mine sub-q because it's easier but I also work in a medical clinic and we give all of those IM. So I guess whatever is easiest. They both work equally well for me.
Next I'm going to try the B-vitamin combo. I've never done this because I felt that B-12 was really the only one that needed to be injected, but I've heard good things about the injectable complex.