Terraria armor is more than a defense number. Many sets support a specific damage class, resource system or playstyle through individual-piece bonuses and full-set effects.
Start with progression stage
An armor set can be excellent for one stage and irrelevant later. Compare options available now before planning around materials or bosses your world has not reached yet.
Class support matters
- Melee-oriented sets often emphasize durability or close-range damage.
- Ranged sets can support ammunition or ranged damage.
- Magic sets may improve mana or spell damage.
- Summoner sets often improve minion capacity or summon damage.
Defense versus set bonus
Higher defense is not automatically better if the lower-defense set strongly supports the class you actually use. Compare total survivability and damage together.
Accessory synergy
Armor and accessories should solve the same build problems. If armor already supplies strong offense, accessory slots may be more useful for mobility or survivability.
Upgrade timing
Do not farm a replacement only because it is technically stronger. Ask whether the current armor is actually blocking progression, and whether the new set will remain useful long enough to justify the effort.
Source: Official Terraria Wiki — Armor.