The more "F's",the finer the powder.The finer the powder,the faster it burns.Most recommend "FF" in muzzleloader rifles.In my rifle I use "FFF" after a lot of range experimentation with both powders because I find I get faster more even ignition with "FFF".
3FG is the faster burning of the 2, more pressure, more velocity, cleaner burning.
If you have a factory recommended load for 2F and you want to try 3F, back off the measured amount by 20%. Notice I said "Measured" not weighed. Not the same thing.
I use 3F in all my rifles. From .40 to .54.
No desire to change.
For roundballs, fffg is usually going to foul less and produce suitable velocities and accuracy in guns of .58 cal and under. For bullet shooting, you really need to try both. In my experience, one will work better than the other in any particular gun and there's no rhyme or reason as to which one it will be. Either one will work good enough for hunting, but one will be more accutate and foul less. Most of "fouling" issues are more a function of the bore condition than just the powder though. I have not seen a factory made gun that would not benefit greatly from a lapping job as far as fouling goes. I'm talking about "real" black powder, not the artificial stuff.