Sig all the way. I have owned both an M4 and now own the 556 swat. There are a lot less parts to it and the gas piston system is great. There is no argument that there are a lot more parts available for AR's though. The trigger is great on mine. The flip up rear sight isn't great but I think you can find higher quality sights. It is expensive but you definetly get what you pay for.
