A Plasma Rifle is like a Laser Rifle. It requires the Microfusion Ammo which is not very common unless you are killing those Enclave bastards and good on you if you are. Plasma Rifles are stronger, sturdier and much more accurate than the Laser Rifle models that you will find. They are worth a hell of a lot more and they aren't exactly easy to come by when it comes time for repairs. In this sense it is worth more to sell it off if in good condition and go for a Laser Rifle instead due to the rarity of the Plasma Rifle and the less rare Laser Rifle. The Laser Rifle is also much cheaper and much easier to repair. In short, if you want to take that Raider down at 50 meters, if you are willing to sacrifice accuracy to take him down quick before he shoots you, go for the Laser Rifle. If you want to take him down with a plasma punch that will kill him quicker then go for the Plasma Rifle, just try not to miss.