Honsetly I don't see much of a point for either one, as I can make those calculations in my head quickly.  I'd rather have my HUD space used for things that I wouldn't know on my own.

As far as which one is more useful to think about (which is probably what you're asking), I would say you should probably just use whichever one you are more familiar with using, because as long as you know the right thresholds for the one you are using, they give you basically the same information.

If you play on a site with variable antes, M is advantaged (do such sites exist?).  M is more accurate for the situation you are in right now (number of players at table change the starting pot size slightly), but BB gives you a better sense of where you stand in the tournament (i.e., it doesn't change when your short-handed table breaks).