I'm about the same size as you (but I think I may have a larger butt). I have one pair of the boyshorts in M and one in L - I would say that the M fits better while the L is slightly too loose. To be honest, both pairs give me a little bit of a wedgie though - has nothing to do with sizing but more with how they are constructed. Basically there is the same amount of fabric on both the front and back sides, and I am quite a bit more... ahem, perky on the back side. So I would say M, but maybe choose a different style if you're curvy on the bottom.