In-game, it doesn't really matter. You should check the rest of your team, and see which you need more coverage for -- Poison and Fighting types, or Grass, Steel, Fire and Ice types. I personally would opt for Fire Fang to cover Sandaconda's weakness to Grass and Ice, but Zen Headbutt is a very solid, higher-powered move and is a very viable alternative.
Note that I would actually consider getting rid of Glare and using both moves, as paralysis is not usually worth a moveslot in-game, especially not in Sword and Shield.
Hope I helped!
