Twink could be MM and do just fine. It depends on which "9" you are twinking at as the talents can shift on you, but at 59 it's kind of hard to say spec wise since you have most major talents at that point. Then it's up to your playstyle. If you prefer a BM style of play, stick BM. If you are wanting to play some other style than your main, go MM. It won't hurt you and both can be effective.
At 59, MM can have silencing shot and some SV talents. You need to weigh that against the BM talents and going into the MM tree for mortal shots. You could even hybrid into all 3 trees for Humanoid slaying if you wanted some MM skills and some SV skills on your BM base but you'd be sacrificing some BM skills a bit. I'd play with the builds a bit before comitting.
Twinks are rather wierd to begin with since it's all about the gear and 59 is a little late in the game to get any clear advantages by gear since you could be up against Outlands stuff. I'm not sure you'll find much twinkage help around here beyond what you've gotten...