The RFB is... Interesting to say the least. But for my .308 I'll stick with something proven, M1A, FAL, G3, AR10, etc. There's others, but meh.
I fired the RFB owned by one of my brothers and wasn't really impressed. The trigger on bullpup rifles just suck. Probably the whole transfer bar theory, or plastic on plastic, who knows. With a .308 you have the range and power to reach out and touch, so why not be able to actually reach out accurately? A shitty trigger is absolutely detrimental to accurate rifles. While the trigger on the RFB isn't technically "Shitty" it wasn't good, and I'd barely classify it as adequate. It was longer than it should have been and creepy. Sometimes I'd mistake a creep for the let off point and it wasn't easily defineable. If they could somehow engineer an M1A's trigger onto that RFB, they'd have a winner.
I voted for the STG by MSAR. I've got one and it's all kinds of cool fun. The trigger sucks, but is adequate for the distances the .223 gives you, and 42 round magazines are pure sex. I'm sure someone's going to point out that you can shoot 1000 yards with .223, whatever. Not out of a 16 inch barrell you won't.
MSAR's service is second to none and accessories abound nowadays.