To be honest, I don't know why everyone insists on trying to even out their physical and magical defense. I usually choose which set to wear based on 2 things; what I'm fighting, and what armor type most of my recent blue drops are

. If you're dueling or doing trade runs, then I can see why you would want to have all-around defense, but if you plan on grinding until the next armor grade, I'd base it on the attack type of the monster, and, following fireandice's tip, whether they're a higher or lower level than you.