Mamoswine Move Preference

For attacking gyms and raids: ice
For defending and PvP: Neither

I've unlocked 2nd charged move for my 100%, so it has both Avalanche and bulldoze. I keep changing my fast move though...

It is the best ice attacker in the game. With dragons being well represented in legendary raids, it is ideal.
It is the second best ground attacker by DPS, behind Groudon. It weakness to fire and steel hold it back in that role. Fire is a very common T3 and 4 boss type.
It is the only Shinnoh that I have evolved multiple (3) to have each type. I'm happy with that decision. If you are only going to have 1, go ice.

Powder Snow + Avalanche is generally better, but your choice should also depend on your other attackers. For example, if you have a good Articuno and Jynx, but no Groudon, Mud Slap + Bulldoze may be more useful for you.

Powder snow+Avalanche is the BEST ice attack combo, bar none. Powder snow fills up energy very fast, and avalanche does loads of damage for relatively small energy. If something does not resist ice, it will die fast. If something is weak to ice, it will die very fast. If something is very weak to ice, well, you get the point.

I evolved 5 very good and high level mamoswines and never got the coveted powder snow+avalanche combo. As an ice type trainer, it bothers me to no end. I can't do raids and I don't have tms. I need to save some stardust for extra attacks on my powder snow mamoswines. But I fear even then I may not get avalanche.

Full ice moveset is the way to go, unless you are lacking serious fire power in the ground department. I recomend having at least 3 ice ones (a little easier due to double candy during com day. A lil hard due to the recent drought of TMs though!