Well, it's rarely really a decision as you can't "stack" armor per se. That said, do not underestimate the value of those necks, rings, and cloaks (and anything else) that has extra armor. It is a very nice way to boost up your effective health.
400 armor vs 1 or 2% avoidance isn't a good trade, but it's also not a fair trade, you'll see itemization trade it more along the lines of 400 armor vs 0.3-0.5% avoidance, which is a fair trade, but between two rather different survival tools.
So, the short answer? You don't need to trade one for the other, take the piece that you like better overall. Piece #1 is Str/Stam/Def and Dodge+Parry, while choice number to Str/Stam/Def and Dodge+Armor, grab the one with the better total (usually the higher level piece).
The more appropriate answer would probably be, between equivalent pieces, with Blood/Frost I'd favor Armor, and for Unholy I'd favor all the avoidance you can grab.