If you're getting hit by multiple types of elemental damage then using a single Resist Magic is better than a single Resist Fire, but there is just no reason to not use potions of Resist Fire, Shock and Frost since the Resist Magic potions are so ****.
Enemies simply don't use non-elemental offensive magic because it basically doesn't exist in this game. There's no Silence so no need to worry about that, never seen anything use Paralyse, there's no Damage Health or Absorb Health (excluding enchanted weapons) spells, its all elemental damage. So really, Resist Magic is totally useless unless you can get it as a permanent effect in reasonable quantities. Plus NPCs tend to stick to a single elemental type and basically all creatures are geared towards one element. I still use a Resist Magic potion along with my Resist Frost/Fire against a Dragon because every little helps in that situation but otherwise... ignore Resist Magic.