there both pretty close when it comes to performance
iv used both (picked up supmods and bought m416s)
the only real difference is the sop has more spray but it also includes a silencer
i pretty much only use an m416 weather NX or GP with an itech do to its lack of recoil and spray at close to mid range (i dont use silencers because it would make it as 5-6 hit kill)