Lange AC9 (MF) v. Lange FR120 (LF)I haven't tried both 120 LF/MF, but had the AC9 MF & have the 120FR LF.
On the forefoot, the LF is narrow enough to have about 1/8" material punched out at the 6th metatarsal (the spur on the outside edge of my foot). THe MF required no punching, but was snug.
The MF required me to have an ankle wrap around the heel/ankle. The LF required me to have a much less substantial "C" applied at the ankle. The LF doesn't require as much cranking down on the buckles.
Also, watch out for moving the buckle too far inward, as this affects flex.