If you're planning to use it on a merc on one character that is of an especially high level, and if you know which other two gear pieces your merc is going to use, so that you are 100% certain of his strength from this moment on and forever into the future, then you can choose the armor with the highest strength requirement and thus which one gives the highest armor:
http://classic.battle.net/diablo2exp/items/elite/armor.shtml
But if you're planning to use the armor on either yourself, another character or another character's merc in the future, then you need to take that into consideration and mabe choose an armor with lower strength requirements but also lower defense. We can't make that choice for you.