Either is fine, really. They're both decent picks for in game play through. Personally I'd pick Froslass because I like to be faster, but the choice is yours.
The two of them are pretty closely matched, honestly. It really depends on the rest of your team. If you need Grass coverage, Abomasnow is better, otherwise Froslass is better.
Froslass is available immediately after getting a Snorunt if you have a Dawn Stone, but it does need the Ice Beam TR to be more effective. It has much higher Speed, and learns Hex upon evolving, which is decent but you'll probably want to swap that out for Shadow Ball once you get the TR. It has fewer weaknesses (5) compared to Abomasnow (8, one of which is 4x).
On the other hand Snover evolves at level 40, which isn't very late, and it immediately learns Ice Punch so it's pretty useful as an ice type attacker. At 43 it learns Wood Hammer which is super strong to use in game. Like Froslass, it has access to Ice Shard and Shadow Ball, so it can definitely hit hard. However, it is much slower than Froslass (60 compared to 110) but in game that doesn't matter too much if your levels are higher. Besides, it has the bulk to tank neutral hits better than Froslass.
Depending on your play style, you might want to choose whether you prefer a faster Pokémon or a harder hitting one. The difference is negligible and they'll both serve you well. I think the bigger factor is whether you need Grass coverage or Speed.
Oh, and in case you want to consider another Ice type, personally I found Mamoswine to be incredibly useful as well.
