Guerrilla Defender is a defense gear perk that improves soft-target, blast and armor-piercing defense at once. It covers the three damage types you face most from FND soldiers.
You take less damage from regular bullets, armor-piercing rounds and explosions. That covers most of what soldiers throw at you.
Yes, it is a strong general pick for chest or head. Use it when you do not know what a fight will bring.
No Time to Spare and First Aid Kit help you recover once the defense has kept you alive.