I love FCEU. It plays really well. I tried RockNES, and found that I couldn't adjust a lot of settings that I really needed with FCEU.
That being said, I'm putting together a console PC to hook up to my TV. The frontend is GameLauncher. It works really great - on the monitor. When I move it to the TV, I can only get Stella and ZSNES to display properly. FCEU won't display properly at all. 1/2 the reason I built this was for NES games! RockNES, OTOH, displays okay, but not well.