Generally speaking, Ice Spinner is always superior to Ice Fang. First off, Great Tusk is usually using Ice Spinner to hit problematic Pokemon like Landorus-T and Garchomp, and you generally want more power to KO them due to their solid defense preventing an OHKO. In this case, Ice Spinner has a greater chance to KO these Pokemon compared to Ice Fang, especially when compounded with previous chip, like from hazards. Secondly, Tusk usually doesn't benefit much from the terrains. The only one that benefits it slightly is when Headlong Rush sets heal from Grassy Terrain, but your Tusk uses Earthquake, so Grassy Terrain also hinders it. On the other hand, Ice Spinner removing Electric Terrain can actually greatly hinder the future Paradox Pokemon. Terrain is only a thing in National Dex formats, though, as the limited terrain setters in the base game means that it's usually only Grassy and Psyshic Terrain, set by Rillaboom and Indeedee.
All in all, Ice Spinner's greater damage alone provides greater value than Ice Fang. You really want this move to hit against the aforementioned problematic Pokemon, and you often want to hit them for as much damage as possible instead of rolling a dice to freeze or flinch (if you go first, that is).
