Id go for Mamoswine, as others have said, mamoswine has better overall stats than Donphan.
Though you’ll have to wait much later to get Swinub, it’s worth the wait. The only thing, unless you plan to make an Ice type team, don’t use a Mamoswine with Snow Cloak, you’ll waste a move to use Hail and then you’ll get only 20% evasion for 5 turns. No, Oblivious is better in my opinion.
Here’s a nice moveset you can use on Mamoswine Ingame.
Ability :- Oblivious
Earthquake - STAB move, high power, nice accuracy. With Mamoswine’s high attack it’s gonna do lots of damage.
Avalanche - I prefer avalanche to ice fang, because, thanks to Mamoswine’s relatively slow speed, it makes him a physical Sweeper.
Amnesia - The only drawback with Mamoswine is his low Special Defense, but you can quickly cover this with Amnesia.
Ice Shard - This might seem like a filler, but Ice Shard can be really useful if Mamoswine comes across a Tank and manages to damage him only to red. Ice Shard saves the situation.