Go with Weavile.
Weavile is just a much better attacker on the offensive side compared to Frosslass because of its superior physical Attack and a bit faster speed. It's also a bit bulkier than Frosslass on the SpD side, but it doesn't matter because in-game is all about offense.
I would strongly argue for Weavile being at least B in Platinum, though significantly lower in DP. I haven't got a full write-up or anything - yet - but it was an incredible instrumental part of my team in my last run. A much earlier Razor Claw in Cyrus' room helps it SO much, and from then on it only really needs Ice Punch, Night Slash and Swords Dance - the last slot's completely free and it has fun demolishing anything that it's not weak to. Incredible against Lucien and Bertha, pretty decent against like half of Aaron, Cyrus and Cynthia (it's the best thing you can possibly get against a Garchomp); in general pulls in great performances against the normal trainers and even neutral matchups like Volkner. I'll try and get something written up but I feel it's very underrated in the current list.
Source
At the time you will catch Sneasel, there should be enough money for you to buy the Swords Dance TM. You also get a Razor Claw much earlier compared to Diamond and Pearl in Team Galactic's Headquarters as a hidden item.
Surely, back in DP when there was no Ice Punch Move Tutor you had to rely on grinding Sneasel until Level 48 so that it gets the weak and pathetic Ice Shard. But, now you can learn Ice Punch via a Move tutor on Route 212 at the cost of 2 red shards and 6 blue shards. You should have that much Shards for Ice Punch and money to buy TM Swords Dance in Platinum.
However, Frosslass can still be a choice if you want to get a Ice-STAB move like Blizzard or Ice Beam via Level Up / TM, though Weavile is still better overall.
Hope this helps!
