So I hit the magnificent 540 def cap number after a lot of mis-mashed blues and crafted purples. I came out of it with about 23k HP. I've managed to crawl that up to 26k HP, but my defense has gone down well below cap 522ish. I'm scared of being below defense cap but it seems like all my pieces with high defense have really low stamina. I'm slowly crawling up my HP, but I'm not sure how to do it without sacrificing defense.
This is my 3rd string alt, behind a priest main, and a rogue alt, maybe I'm just not investing enough into gems/enchants etc.
My question is, is it ok to sacrifice defense for HP till my gear has enough defense already on it to hit defcap easily?
Heroics can be successfully tanked with 18.5k HP and some work on the part of the healers, and this is said with first-hand experience. If you were at 23k, you were at a great place while you were above the 540 def mark. There's a smudge of wiggle room in your HP pool, and you won't be demolished with crits. All you gotta do is explain to the group that you're a pretty fresh 80, and find a group that's ok with this. You'll likely go through a few overgeared elitist jerks before you find a group that's accepting of your fresh 80 status, but that's one of the "wonderful" trials of being a fresh 80 tank. The other, especially in regards to DKs, is that we feel really squishy until about the time we're ready for Naxx.
Long story short:
Def > Stam, and at first gem even amounts strength and stam, unless the socket bonuses are highly beneficial. Just make sure you can hit your meta reqs. Your meta should be the Austere Earthsiege diamond, and enchant every pair of gloves you get with Armsman for even better threat.
Best of luck, and happy tanking!
PS - Armory links are very, very helpful. We can zero in on problem areas, and give more specific advice if we know what you're working with.
...Thoughts of a philosophical Death Knight...
If death is the inevitable end of life, is life the inevitable end of undeath?
Helm: 37stam+20def Arcanum of the Stalwart Protector - Item - World of Warcraft
Cloak: 16 Defense Enchant Cloak - Titanweave - Spell - World of Warcraft
Bracers: 40 stam Formula: Enchant Bracer - Major Stamina - Item - World of Warcraft
Total 77 stam, 36 def
Shoulders: Since you don't have Hodir rep yep get Greater Inscription of the Gladiator - Item - World of Warcraft 30 more stamina, and that added resilience will lower your defense cap for raids to 535.
Titanium Earthguard Ring - Item - World of Warcraft should be a fairly cheap solution to your DPS ring.
+54 stam(with gem) +25 defense
Those all all pretty easy upgrades without having to set foot in a dungeon. (Thats good because it means they are guaranteed, not a chance to drop that you grind for weeks).
As far as your actual question, it its preferable to heal a slightly smaller pool that takes more constant damage, than a slightly larger one that has a chance of taking unnecessary spike damage. In other words, stay defense capped.
Vagus I completely understand your frustration, as I myself am I new lvl 80 tank...Trust me when I say it all comes with time. One of the things I've learned with tanking at level 80 is there is going to be a constant enchant swaps, and gem swaps to make sure you are around all of the appropriate levels.
When I hit 80 my defense was at 542, but my dodge was lacking at 15%. Most of the tanking gear at the beginning item levels of 80 has less defense then the leveling gear (I.E Daunting handguards and legplates) but it's includes dodge, shield block and all of that wonderful stuff. I have been having to enchant defense to things such as my shield, or my chest piece to make sure I stay above the cap, but after a while it gets easier! Most everything just comes with time and patience...I have less then 24 hours played time on my level 80 tank and I am doing very well for myself...its just heroics heroics heroics to get to the 226 item level.
I am not saying I am the most idealistic tank to gear after because again I am still very new at it myself, but you and I are in the same general position. Check out my armory for a few of the things that I went with to help with being more accepted into instances / raids.
Noob Prot Pally - Armory