Unless you aim to do Firestorm Citadel in the very near future, I wouldn't worry about the metagame influencing your armor choices.

I consider Elemental defense the most useful because of the current prevalence of Gun Puppies in just about every existing level.

As for Piercing damage, only spikes and spiky slimes do that for now. And now I can't remember if flamethrower gremlins actually damage you with their fire, or just set you on fire. They do occasionally shoot out elemental-damage energy balls, though.

Retrodes can attack from long range with a powerful laser. They might actually be LESS dangerous at close range, because their melee attack is easy to block.

The fire from Gremlin Scorchers inexplicably deals Normal damage on all tiers. I will never understand why it's not Elemental. Same with the bombs from Gremlin Demolitions.

Also, there are almost no enemies with Piercing attacks. The only one you're likely to meet regularly is the Lichen Colony (and in most cases this is preventable-- just don't let them merge). Kozma's armor recipes are... kind of a trap, really.

If you're some kind of ranged dodge-master, then you'll want to think about Shadow defenses. Many Shadow-based enemies can quickly close the distance (Spookat, Zombie, Greaver, Phantom) or attack from range (Spookat again, Phantom again).

Elemental attacks are still more common than Shadow attacks though.

You can use your costume slots to have the looks of a piercing-based armor but the defenses of an elemental-protective armor.

If it's the Gunslinger outfit you wanted, it's cool-- go for it! There's an upgraded 5* version with Elemental defense (Nameless set). It swaps out the damage bonus with a considerable attack speed bonus.