Competaively speaking, I, despite my love for Swampert, would say Feraligatr is superior to Sampert.
Sure, Swampert have a better typing, getting STAB Earthquake to play around with. But Water/Ground is a dual edged sword. Both Swampy and Feraligatr have 4 resists, but Feras singly type makes it less vulnerable to Grass, sometimes allowing it to live one hit or two from a Grass move. Of course, its not ideal being pure Water, but it have its perks.
Statewise they are almost equal, albeit they play very different roles, and this is where Fera will start to shine, because it have one quality few Water types have.
Swampert is in every sense of the word a tank, being bulky whilst dishing out the pain. But it's lack of reliable recovery and few resists severely cuts the time it can stay alive. If you are looking for a bulky Water type, there are many better options, such as Jellicent, Milotic, Slowbro or possibly even Quagsire. Sure it have Stealth Rock, but otherwise it doesn't have much going for it. Feraligatr on the other hand, can dance!
Like that ^
Access to both Dragon Dance and Sword Dance allows it to set up to dangerous levels. It may be unable to hit some things dependant on the coverage moves you choose, but it is definitely capable of plowing through teams given the rigth circumstances, something Swampert lacks (it may have Bulk Up but that's no real setup to talk about). What I'm really trying to say is that Feraligatr is much better at its job (setup sweeping) than what Swampert is at its job (tanking).
So in conclusion, if you need a bulky Water type that's not weak to Electric Swampert is your man, but if you want a dancing alligator of doom, Feraligatr is a better pick.