All forms of metal are vastly better than cloth and leather against all forms of attack. Leather is better than cloth against blunt attacks, while silk is better than other forms of cloth and leather against all forms of edged attack. No form of armor is very effective against blunt attacks anyway, but metal is so much better against edged attacks than even silk that using silk on top of metal for protective purposes is basically pointless, so you're better off going with all metal and leather. Practically speaking, any metal armor blunts or blocks edged attacks from any inferior material, so using cloth for edged protection is a waste of effort.
If you have armor-grade ores, you generally have the associated metal in vast quantities, so you can typically equip every soldier (or even every citizen) with metal armor on every body part. If you're reliant on imports or goblinite for metal, mail shirts and helmets cover the most important areas. Instant-kill attacks are those that hit the head (which the helmet defends), that hit the heart or lungs (which the mail shirt defends), or that cleave the lower body off, so the third-most-important piece of metal armor should be greaves to guard against attacks that sever the lower body. Shields protect all body parts better than any armor when wielded by a skilled dwarf, but can be made of wood in a pinch, so they're not really a concern for metal conservation.