Blacks are the best FPS switch but many people choose reds as they work better over a range of applications - unless you want to keep switching keyboards for different uses you will lose on a very slight benefit going to reds over blacks in regard to FPS performance but gain a big increase in useability for other games and typing.
Also as above some people also prefer the reds due to being a bit softer in use but thats generally at a slight trade off on actuation precision (depending a little person to person).
EDIT to put some kind of numbers to it - purely for illustration purposes:
Blacks: FPS gaming 100%, other gaming 50%, typing 40%
Reds: FPS gaming 95%, other gaming 70%, typing 60%