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There are two levels on which I'd answer your question:
1.) The short and simple answer is that each of the trees has particular mechanisms that it relies on and works best with a particular focus. Blood has many mechanics that focus heavily on total health. Frost is more balanced and can benefit from all the mechanics, though mitigation, armor, and health are most supported by the chief CD, Unbreakable Armor. Unholy benefits most from heavy avoidance, though you do need to be careful not to shortchange health as when Bone Shield is down you need to be able to take a heavy hit. As I see you are Frost spec'd, I think the choice is really on you, though it would not be a bad idea to pick up extra armor where you can, and Frost can get away with the least pure health stacking since your protection talents focus on reducing incoming damage (not that you should avoid it entirely). Interestingly, avoidance doesn't play as nicely with Unbreakable Armor since it reduces what the CD actually does, but the avoidance still means you have better survival.
2.) The way gear is now you get a fantastic and pretty well-rounded baseline on all your survival tools (health, armor, avoidance, ~expertise) straight from gear, so you can't really mess it up with what you gem. In general, I think the best advice is to focus on a theme (best supported by your spec is smart though not necessary), and make sure you don't completely neglect the other survival mechanics. In other words, it's fine to stack Stam heavily, but if you can get a better value from one trinket slot, a gem slot or two, or your rune instead of just stacking a lesser value for the sake of pumping every ounce of health out of it, that's the better choice.
In a general theory sense, the difference between stacking avoidance and not is probably about a 4-6% swing in your avoidance next to a 4-8k swing in health. Neither will make or break but you will see a distinct difference depending on the fight. Higher avoidance will mean more gaps where you won't take damage, but lower health means it takes fewer hits to kill you. On the scale we're at, and with your experience in Ulduar you probably know where that makes a big difference (XT hits you for 20k, if you have 45k health you take 2 hits in a row without dying, with 38k health the second hit kills you).
There is no universal maxim. Personally, I've been trying to keep my options open to fight to fight gear switching. I switched around gems and started using Rune of SSG so I could swap both trinkets freely without becoming crittable. I'm Blood so normally my focus is health, but on fights like XT I swap in more avoidance so that I can more reliably not get hit too many times in a row (there are bosses and scenarios where you can't buff your health enough to take 3 hits in a row without CDs or heals).
Does that help?
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