Walrein or Lapras

Not "impossible". If OP is not specifying level, assume they are the close to the same level (my default is to assume level 20 egg hatches) or OP has enough stardust and candy to burn on either.

Or include that in your answer "Foo is better than Bar, but they are close so if the other was caught at a higher level it can do."

Walrein’s access to only one-bar charge moves does kind of stink

Disagree - while Lapras is a tank, if you want to use it for raids, Walrein has higher DPS and lower tankiness, which allows for more total damage. But they are pretty close in terms of the overall stats.

And piloswine evolves into a beast next generation :3

AND piloswine is
- neutral to ice and rock,
- is immune to electricity
- but weak to water;
whereas the water/ice hybrids
- are weak to electricity,
- double resist ice (the same as an immunity essentially)
- weak to rock
- resist water

But yes the points you have made supersede mine in terms of relevance

Walrein has high base attack, and is still fairly tanky. There are several other ice pokemon with similar attack but are a lot less HP or are legendary.

Lapras is even more tanky, but has lower attack. When exploiting a double weakness for attacking a gym, Lapras' attack is good enough and allows for less healing with one max potion at the end. For a raid boss weak to Ice, Walrein with higher attack might serve better, but you are going to need more attackers anyway and Lapras's tankiness might help prevent the dodge bug for longer.

But Lapras is outclassed big time as a water type (Walrein too, though), but not as an ice type. You shouldn't really talk about the ice/water hybrids as water types.

OT: I'd say Walrein is better due to its stats. But that might be my opinion as I never had a proper Lapras to begin with and therefore didn't get to test it...