+20 defense is a better overall enchant, if you think about it in gemming terms, you could have 1 epic gem worth of defense, or 1 green quality gem worth of stam. In that department it's a no brainer. However, stam is very important in ToC and obviously getting to acceptable levels is the first step to being able to handle a progression boss.
It comes down to the individual. I just broke 40k unbuffed hp as an orc warrior tank. To me, and to my raid, this is more than acceptable for the fights we've been working on, and as such, the extra avoidance I get from defense helps smooth out my incoming damage since I can survive the burst that happens to me. Your mileage may vary.