Defense doesn't matter so much within each level of armour, but you certainly shouldn't keep your current level's armour once the next level is available. With upgrades, the max defense of each level's armour is nearly identical, so yes, as Rbtr said, pick a suit of armour for its skills.
Mistakes are made, I've never seen anyone beat a monster without getting hit. If you're using leather armour against, say, Uragaan, you're going to get wiped out in one hit.