For standard case cooling, I'd go with the SP. If you're water cooling and need to push air across a radiator, then I'd go with an ML for the increase in static air pressure.
Apparently the ML are louder by 2 dBA however. Not sure how noticeable it is unless it's running full RPM.
You can go with ML regardless if it's not too much of an added expense. If you're on a tight budget, and won't be water cooling, then go with SP.