I am not sure if there is much difference but one might be better than the other.
Hail, because if you have the ability Snow Cloak on your Mamoswine then your evasion increases during battle. Or because fewer Pokemon are immune to Hail.
If it has Snow Cloak, Hail. If it has Oblivious, give it a better move like Ice Shard or Avalanche/ Icicle Crash.
If this is for competitive play, neither are good moves for Mamoswine.
