Because there isn't one. They released a bunch of different models - all with similar performance, mind you - likely to deliberately cause confusion.
Radiator fans: LL120 2200rpm version. That's it. The others don't have enough 'umph' for practical use on the typical hybrid cooler radiators.
Chassis fans only: It matters little, though the LLs and QLs appear to have more LEDs in them(that ring) than the MLs and SPs. That may make them more desirable for you.
QLs were designed to be appealing from both sides - the other 3 have their accents mainly focused in the front.
140mm fans should move more air while being quieter than 120mm at a given noise level...
Honorable mention: the HD series. Those would've been a contender, but Corsair...