It honestly depends on the spec.
As a frost mage, I'd certainly use ice armor, not all the time, but namely in duels against warriors or rogues. Why? Because the slow is simply priceless.
However, molten armor's 10% proc on the stun is also great, not to mention the almost 3% resilience, as well as the crit chance, and the occasional crit to inflict an ignite to prevent stealth for 4 seconds.
If you're a frost mage with permafrost, I'd go ice armor definitely against any melee class. But molten armor pretty much beats over Mage armor, as it gives so little now outside of raiding.