Mamoswine:
Pros:
>1. An amazing attack stat of 130
2. Thick Fat gives it a lot of resistances and chances
3. Good priority in Ice Shard
4. Wields one of the most powerful Earthquakes in the OU tier.
5. Good offensive Stealth Rock lead
6. Functions as a suicide lead with Focus Sash perfectly fine
7. Good Anti-lead
8. Can fit in on both sand and hail teams
9. Has a good array of choices (items; can weild both scarf and band for usage)
Cons:
>1. Weakness to common priority in Bullet Punch, Mach Punch and Aqua Jet
2. Subpar defenses
3. Base 80 speed is average at the most, and those Pokemon it cannot defeat with Ice Shard pose a problem to it.
4. Limited movepool
Frosslass:
Pros:
>1. Ghost type got a huge buff this gen, working in it's favour
2. Has access to a lot of support moves like Stealth Rocks, Spikes, Hail etc.
3. A good base speed of 110.
4. Cursed Body can potentially cripple a physical attacker.
5. Excellent siucide lead in the lower tiers with Focus Sash
5. Can keep it's hazards from being spinned away
Cons:
>1. Subpar defenses
2. Next to no offensive pressure
3. Stealth Rock weakness limits maneuverability
4. Defog makes it's life more difficult.
5. Ice/Ghost typing gives it a lot of common weaknesses, including but not confined to: Dark, Ghost, Fire and Rock.
My Opinion:
I personally thing that it depends on the role you are looking for. As a suicide lead and spin-blocker, Froslass can do a better job than Mamoswine. However, offensively, as well as generally, Mamoswine is much better.
Hope I helped!
