Well in my opinion the 416 and the m4S are the way to go, get some paint and enjoy your new gun that will never weigh you down while in game
That said, you seem to like the heavily railed guns. Is that what you want? Lots'o Rails? In that case the M4S has two stubby rails and a third longer rail with full through top rail. The 416 has full rails on the sides and bottom, and though it looks like two peices, I bet the top rail is full through aswell.
The SIG is a pain to work on for the series, but I have not seen the 556 in its STANAG ready reciever. It would be a world of a difference to me if it was easier to dissassemble. About S8, call them get your questions aired out and forget these people that are speculating. They are the local shop here and they treat people right (paintballers can be good people too).