I would definitely chose Swampert.
It have a more usefull type-combo, first and foremost. It may be 4x weak to Grass, but that is the only thing it is weak to on the other hand. Torterra have a similar 4x Ice weakness, but also present a weakness to Fire, Bug and Flying, all quite common types. Offensively, Torterra don't fare any better, having slightly worse coverage with Grass then Swamperts Water type.
But it is in stats the Swampert start to really overtake Torterra. It is stronger in every stat except for Defense, and combining only 1 weakness with 100/90/90 defences make Swampert a formidable tank. Torterra suffers from its bad Special Defense, making even a neutral Attack hit harder than desired. In a 1 vs 1 battle, Swampert would always win due to its higher Speed and Ice move coverage. And one move Torterra desperately wants to use, Giga Drain, is rendered useless due to its pathetic Special Attack.
In moves, they are very similar, both having access to powerful STABs, coverage moves and support. Torterra is slightly more versatile on the offensive side, sporting powerful moves such as Superpower and Outrage. But despite this, Swampert is better at the job of being a Tank, since it can hit hard, phaze and set up. Sadly though, both lack any reliable form of recovery, so they have to rely on Rest and Leftovers to regain health.
Here is the set I (and almost everyone else) use for Swampert:
Swampert @ Leftovers
EVs: 252 Atk / 252 HP / 4 SDef
- Ice Punch
- Stealth Rock
That would be all, hope it helped.