I have had all three, the ksg is a novelty gun in my mind, switching tubes is a hassle and not a good feature and jams are hard to clear, the mossy is unbeatable although its capacity is limited, I have had a gen 2 uts15 and now a gen 4, absolutely incomparable, the new gen 4 is amazing, 15 rounds fired as fast as you could imagine, low recoil, absolutely flawless function, plus its easy to clean and if you had a jam, which I have not, its easy to clear
UTS15 all the way