What level does Snorunt evolve?

level 42
Snorunt evolves into Glalie at level 42. A female Snorunt can evolve into Froslass by useing a Dawn Stone.

Is Froslass or Glalie better?

For PvP, make sure to grab Froslass. If you want to maximize the number of points you can earn with a Pokémon, potentially using it more often in raid battles, Glalie is slightly better.