Let's compare their effectiveness:
STAB moves super effective against Malva, Drasna, Wikstrom, and part of Diantha's team. It does get slaughtered by Siebold and Malva, however.
STAB moves super effective against part of Diantha's team, and resists Drasna's Dragon moves, as well as her Dragalge's STAB. It does get decimated by Siebold's Clawitzer, Malva, and Diantha's Hawlucha.
Overall, I would say to use Mamoswine. Here's a potential moveset:
Mamoswine @ doesn't really matter
Ability: Thick Fat would be ideal, but you probably can't get that.
-Earthquake (Level 46)
-Ice Shard (Level 24 as a Swinub)
-Stone Edge (TM in Frost Cavern) / Rock Slide (TM in Couriway Town)
Hope I helped!