Early game, antirifle vests make most small arms bounce off of you. Late game, even 100 DT will do nothing vs 1000 critdamage snipes.
Metal armor can get some sick high% DR. For the other slots, consider your build.
Metal helmets reduce Perception (and thus CTH). For assault rifle builds capable of unloading a magazine in one turn, this isn't a big issue; you'll kill most enemies anyways, and the with a high quality crafted metal helmet you can come close enough to capping your mech/heat/energy resists (assuming high quality metal armor) you are basically invulnerable to those types of damage.
But this would be a big no-no on a sniper rifle build; misses are so frustrating you really need to have NVGs on at all times.
Metal boot Striders can make you very mobile, but metal/metal boots have sick mech/heat/energy resist. One issue is other resistances; with a padded metal helmet you get either electric or cold res, with infused mutant dog Leather tabi you can get a ton of acid res. Making dog Leather boots, unfortunately dramatically cuts the acid resist.
Still, if you place armor into all 3 slots you can be invulnerable to mech/heat/energy, a decent resistance to Bio (if bio vest on metal armor) and acid (if infused mutant dog Leather tabi), with a small amount of either electric or cold resist (depending on helmet padding).
This build has weak electric or cold resist, and completely lacks another. A variation is using boots instead of tabi, which lets you grab an extra set of padding, but greatly cuts the resistance from Leather material;not recommended (unless you love Striders. I do.)