Stone armor has great defense, a small all resist bonus and gives you a huge hit recovery bonus. A good merc armor for relatively cheap.
Fortitude armor has good defense, a decent all resist boost and a juicy 300% enhanced DAMAGE mod which is multiplicative, and even more defense from the chilling armor. Plus you get a 20% average damage boost from critical strike.
So if for example you have a weapon that does 100 base damage with 50% ED on the weapon (150 damage), then add Fortitude armor, that 100 base damage would do 600 damage.
Frostburn Gauntlets have a 5% enhanced damage mod as well, it’s also multiplicative.