Actually, both are pretty bad. They are extremely slow, and water-ice is a terrible defensive typing because it is weak to 4 types, including Stealth Rock, but only 2 resistances- itself. You could argue that it is a decent offensive typing, but Cloyster is a way better Pokémon at that, as it has access to Shell Smash and a better ability in Skill Link.
Cloyster @ Life Orb/Waterium-Z/Focus Sash
Ability: Skill Link
EVs: 252 Spe, 252 Atk, 4 SpA
-Ice Shard/Rock Blast
What is better between the Pokémon you mentioned? I think Lapras would be slightly better in this case. Unlike Walrein, Lapras has access to Freeze Dry, allowing to somewhat check Water types. Its Water Absorb ability compliments this further.
Choice Specs Lapras is best as it allows it to wallbreak.
Lapras @ Choice Specs
Ability: Water Absorb
EVs: 252 SpA, 120 HP, 136 SpD
-Hidden Power Fire