Both are good
If you evade you take no damage at all
With defense, you simply take less damage
Unless you can hit 100% evasion, or 100% nulification of damage, you're usually best off strikeing a balance between the 2
Also play style comes into play as well, if you find your self relying heavily on evasion abilities, then evasion gear will likely aid you a little more.

I know in some games players like to break everything down into a formula and take the fun of individuality out, just to prove that their setup is better. All they do is suck the fun out of it. ^^

With all the coming changes to the armory system, new gear, and changes to classes, I say, have fun, pick out some gear you like and have fun. ^^

As for is it better to have both, definitely, some mobs do magical aoes, some do physical, and some just hit no matter how good you can evade ^^
Knowing your enemy is one of the best routes to success.