The Carnage and the NS-11A are both good guns. The NS-11A is really easy to shoot and can be set up for any range, and the Carnage is great up close and stretches out to medium pretty well. For me, though, as a free player, neither is enough of an improvement over the Gauss to warrant the spending of certs that I could be using for class abilities, utilities, revive nades, etc. I think the Gauss is a very underrated gun. I run it with a laser sight and a 2x reflex, and that works pretty well at any range. The only thing I really miss is the .75 ADS move.
My choice: MKV Suppressed paired with Blackhand revólver,u have weapons for all clases and factions and all round weapons for all distances.