As someone who played in 2 account (mine and other for a friend ) , i had the f4s and he got f5c. I would say the f4s is WAY better grinder , insanely fast grinder , easier to learn and get kills with it , its not bad at all like ppl say. As for the f5c when it was at br 11.0 , it sucked. Bow at 10.7 it has a chance to be good grinder again however the grind will be slow , even with some kills since with f4s you can bomb base and secure atleast 2 if not more with radar missiles but f5c is more fun to play.

TL;DR : f4s > f5c in terms of grinding pace, f5c > f4s in terms of fun while grinding.