I dont use ether of them in my built C4.
I use B&M trick shift for years now with no problems at all. it runs cooler and I get very very little dirt in the filter and in the pan.
B&M Trickshift is approved for Type F/Dexron/Mercon. They put additives to cover all common fluid types. Certain clutch material require different additives. I myself would use B&M only because it has a better federal fluid rating.
Some fluids have additives the actually let the trans slip to soften gear changes. There are people running Mobil1 5w/20 oil to make the trans slip less. Some people are using hydraulic fluid for the same reason. I've heard a few places that different weight oils can loosen or tighten the TC up to 300 rpms as well. I'm guessing that they recommend that trans fluid because it does not slip as much. Call the tech line.
The difference is the addition of friction modifiers, Dexron and the Mercon fluids have friction modifiers in them to make the gear change and torque converter lock up smooth. Think of a full size Buick or Lincoln, most drivers of these types of vehicles would object to a firm quick shift, they want that smoooooth transition to the next gear.
Type F has little or none of these additives and as a result the shifts feel firmer. The viscosities of these fluids are the same, it just just the stuff they add that makes them work differently.
Mercon is no longer a recognized fluid, Ford now recommends using the newer Mercon V fluid and GM has replaced Dexron with an upgraded fluid as well. More modifiers for smoother shifts.
I called Lentech yesterday because my AOD makes an annoying noise. Sounds like when you slowly let air out of a balloon- that squealing noise- happens at any RPM, whether the car is in motion or not. The tech guy said to drain the Mercon/Dextron and try Type F to try to get rid of the squealing.
Found out Pep Boys near my house no longer carries Type F...but the dickheads carry crap I don't need from an auto supply store (camping gear, Chinese scooters, etc).