Unless your C is a huge underdog compared to your opponents', I always advocate 1-3-1 against Motion. I can't think of another example when two different tactis create such a mismatch. It's true that they will have 20 offfensive rebounds instead of 5-7. So what? Their shooting percentage will almost always tank under 20%. I normally always let their C get 30 rebounds, as long as they shoot something like 17-95.