I say seventeens for sure. I got 17x9's with my 40" MTR's and love the stance. seventeens will help make your rig feel a little more stable too. Taller rim means your sidewalls wont squish to the side so much when you are off camber. They cost a bit more, but I would recommend them, plus i think anything over a 37 on 15" rims just looks funny!
X2, Big tires with 15's seem to want to bounce more as well which is harder on parts.
