For Frost Armor (7% haste) versus Molten Armor (5% crit), the crit scaling factor need to be 1.4x the haste scale factor.
7%/5%=1.4.
If haste scale for 2, then crit need to scale for 2.8 to make it useful.
I actaully had the samt thoughts the other day and I got a good answer about the armor.
The frost armor scales multiplicatively with our haste and molten armor only scales additively with crit, therefore we always use frost armor and reforge into more crit when cap is reached.