Isn't as great? Based on what you are saying, you are taking a blind-eye by looking at the stats alone without even trying. If you have the weapon and tried it, it can't be helped, but you are make preassumptions.
Do note, you best get every weapon, even if it's for the affinity and rank-up. Especially for Primes, I advise keeping them unless they are the much easier ones to farm.
i like the soma... so i want the soma p... if the original is great.. and the primed version is (duh) better... doubleing most or all of the stats... then it IS A MUST to get.... amprex has better chance though.... but soma has ammo and range..... so if it gets crowed.. or is sure to do so.. get amprex....put it on.. etc....if its speed your after... not survivability and length....then soma... unless your length means you need to get outta there.. and theres not other ppl mowing the hoards down.... then soma p....whatever
Soma prime has a significant damage boost over non prime due to the way that this primary works using a crit build (base damage multiplied when hitting with a crit). It also has an insane amount of ammo so you can burst fire happily and then when sh_t gets real... hold down the triger to clear a room.
Issues are with ammo economy for longer survivals and the volume of hits needed to erode high health enemies, this can be partially addressed with ammo packs. Running with a good mod setup and as with most primaries - investing a little time and love into the weapon will give you an outstanding well rounded bullet hose.
Is it worth getting... yes, and does stand up as an excellent auto rifle.