Well since your running a MSR I would stick to a moogen bc, just to be sure you get the best mesh. What mill are you running? If you have a mill that likes to rev I would to with a 13, but if you have a stump puller I would go with a 14. I ran both 13 and 14 in my LSP and I've gone back to the 13 because I don't have the best mill I don't want to over stress it. If your getting one of those Nova's I would go with a 14 and maybe even a 15. Those are just nuts!