There is a lot to the BCG discussion more than just reliability. Heavy bolt carriers are stronger and less prone to failure, although it's not a part very prone to failure in the first place. They also help to slow down cycle time in over gassed guns running carbine or pistol length gas systems. However, more mass moving around equals more recoil. That is why you will see some competition guns built with very lightweight BCGs, but then in a standard gas system you will get extremely fast cycle rates and the possibility of bolt bounce. So then you need to start thinking about mid or rifle length gas systems or adjustable gas blocks and heavier buffers.
Bottom line, on a standard gun, I would run a FA BCG