I donno, it reminds me when a kid simply asks me "Pancakes or waffles." To which I reply " Waffles alligator mouth to the left pancakes."
Pmr 30 is a 22 mag the 5.7 is a little bigger round and a lot faster muzzle velocitywith an effective range of 1100 ft if I remember.
The PMR 30 is a Kel Tec and the FN 57 is an FN
They both share many advantages but the deciding factor is budget. Can you afford $1100 for and FN or $400 for the Kel Tec?
If budget is not an issue why are you considering the Kel Tec?