Well, sthalik is about right. The difference in damage per shot between SMG and AR is not that great, while SMG has ability to get more shots per round. A LOT more. The only downside of this is higher ammo consumption. Also SMG can be created silent which offers ability to pick enemies one by one in the area, that by itself is also a huge boost.
SR has ability to basically drop anyone in one shot for a great distance. Sniper has downside of being able to take only 2 shots per round without adrenaline, but those shots in most cases are certain kills and you also has a distance advantage on most of your enemies (often they have to take at least one turn to get close to you). Also SR has lowest cost/effect ratio, you only need 4 feats (Aimed Shot, Snipe, Critical Power, Sharpshooter), 5 Str, 10+ Per and 3 skills (out of which only guns and stealth needs to be maxed, while mechanics just needs to be at a decent level) to get almost everything this weapon has to offer. That is why it's VERY easy for anyone who has SR as main to decently invest into 1 or even 2 secondary weapons (usually SMG or pistols ot both). Sniper build is about the only build that gives me liberty to take some quality of life feats without any regrets.