Why would you buy a single shot variant of the SAW? The SAW shoots so dang slow it's basically a single shot weapon already. Yeah, in beta it was an extremely good weapon on par with the Orion and the CARV. Then enough people complained that it was somehow OP (While completely ignoring the vastly superior EM-1) and SOE decided to make it a poor man's tickle stick. NC always OP in Forumside 2.
Not really worth buying in my opinion I actually had more luck with the previous version than i do with this one. Instead of fixing the extended magazine they just changed the description, it was "Doubles ammunition per magazine" and is now "Adds 4 rounds per magazine" which still leaves a spare shot if you use the burst fire on the jackhammer, which just seems sloppy to me. It also still doesn't increase your piddly 30 round ammo count, and you still respawn with 6 shots (so you have to reload to get the full 10, then re-supply because you have so little ammo you need that extra 4 )
Gauss SAW S got almost every attachment there is. Basic SAW don't have compensator for example (if I remember correctly). Also SAW S have soft and high velocity ammo.