Q: what's the difference between the key of f# and gb?
A: there is no difference in the way they sound. but if you write them down you have to choose whether you want your life to be insane with flats, or insane with sharps. sometimes you can base your decision on whether or not there is a possibility of additional sharps or additional flats occuring (beyond the original 6), so you can possibly stave off further pandemonium