Depends.
What are the other chords?
Am to GM7 has a nice sound to it (Am7 to GM7 as well). So F# there.
But if you're doing something like am - G - F - E the F natural is going to sound more typical.
I'd say, if the piece is in "minor" specifically, use F. If it has more of a Dorian flair, use F#.
Minor is tricky because the minor we play today isn't quite like traditional minor. We tend to play a "modal" minor, or Aeolian a lot of times to prevent it from sounding "too classical". Likewise, we tend to use #6 in things with a dorian flair, rather than in "plain" minor.
It also depends on the style. If a Jazz player is going to add a 6 to a minor chord, they'll add #6 - a-c-e-f#. You wouldn't want to play F over that! But if it's a rock progression, with F chords or Dm chords nearby, you don't want to be messing with the #6 (could sound "too jazzy").
HTH
Steve