I don't know anything about BMC geometry, but do know something about geometry. Given the general principle that you shouldn't jack with saddle position to shorten or lengthen your reach, it sounds like you'll be better off with the smaller frame. That way, you can set your saddle up with respect to your knees, which is what you want. You may find that you'll have more power. You can lengthen your reach with a longer stem if necessary.