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It largely depends on if you have problems staying alive. If you find yourself dying a lot, you should opt for the Shell, which has increased defensive capabilities. If you can hold your own, go for the Scalemail, which gives you extra offensive power.
Go for the Beetle Scale Mail, which gives you an attack boost while you're causing damage. Bosses and invasions have plenty of targets with which to build your attack power. More attack power = things die faster. If you use Vampire Knives, you definitely want the attack boost, because the increased damage translates into increased healing.
Beetle Shell only builds up defence while you're not being hit, so I find it kind of pointless. There are better accessories to boost defence, or you can modify accessories to Warding. Sure, the Shell might save you if you got hit by the Moon Lord's laser hitting you for 150 points of damage... but it's also going to take effect if a Slime nibbled your ankles for 1 point of damage, thus resetting the defence bonus. Terraria is not really a game where you can tank attacks.
Personally, I also keep a set of Turtle Armour for general exploration too. The thorns effect is good for taking care of weaker enemies, leaving you in peace to mine or build.
I find the Shell more useful. With maximized defense you can actually tank the Moon Lord (normal mode only) standing still (as long as you put a layer of blocks above you to block the Phantasmal Death Ray). His other attacks won't do much when you have close to 100 defense. It's cheap but works every time.