Actually, armor reduces damage from ALL damage sources, resistances only reduce dmg to their particular resistance. Resistances and Armor are pretty much equal in dmg reduction.
Intelligence increases all your resistances slighty, so it effectivley is like armor, which also reduces dmg from all sources.
For more info about armor and resistances and the like, I'd read this great post I found where someone did a lot of math about it:
http://www.armadagaming.com/showthread.php?459-Armor-Resistances-and-Effective-Health