FFF burns faster and cleaner than FF...You should also use less as it creates more pressure...Buy using less, you have less residue...Over half of black powder is not burned, some goes out the end of the barrel and some stays in...By reducing your charge, you reduce fouling...

When I started, back in the '70s the rule of thumb was use FFF in .45 and below and FF in .50 and above...We now know this isn't true...

Now, some guns show a preference for one over the other, if so, used what she wants...