Gremlins and constructs both do elemental damage (plus some normal damage). Ash Tail will protect against more of the normal damage, while Magic will protect against more of the elemental damage.
You seem to be into swords. If I were you, I'd just use the Ash Tail set, to get more sword bonus. Buy extra defensive armor only if you find yourself dying a lot. But consider making Vog Cub (rather than Skolver) your first 5-star upgrade from Ash Tail, to finally get some elemental protection.
I'd say the Magic Set. 4* items get nerfed to only slightly better than 3* items in tier 2 (In tier 3 they are back to their normal strength).
I suggest getting a Magic Hood, and then running it with Ash Tail Coat. Upgrading an Owlite shield will be really useful, especially late in the game. You'll benefit in the long run having at least a few pieces of armor and a shield that can defend against each damage type.