When you face this decision, you can and should go read all the pick order guides you can find. There's a couple of excellent ones on Tanking Tips. We'll forgive Vene (Tanking Tips author) for being a Red Bar. He writes some very good tanking articles and if you are a serious tank his blog ought to be part of your reading. Just ignore the Warrior specific stuff.
But after you read the guides, and before you go plunk down your pretty Emblems, remember this:
Tank, Know Thyself!
The first thing I'm going to consider when deciding on a piece of 'badge' gear is looking for weak points in my current kit.
As I examine my tanking kit, I can see that I'm weak at the Bracers (Tempered Saronite), Back (Cloak of the Enemy), and Libram (Tome of the Lightbringer, my last piece of tBC gear). My Neck, Boots and Legs would be the next week points after that.
I can get a new Libram and Neck from Emblems of Heroism. I would need just 5 more Emblems to be able to buy both. So buying the Libram of Obstruction goes on the To Do list, as well as getting 5 more Emblems of Heroism for the Chained Military Gorget.
Bracers, Back, Boots and Legs are all from Emblems of Valor. My Bracers and Back are about equally weak. I only have 25 EoV. The question is, do I buy the Platinum Mesh Cloak now or save up for the Bracers of Dalaran's Parapets.
The Bracers are technically BoE, so I could try to find someone with more Emblems than they need and buy it from them. I've got to imagine that would set the ole bank account back a pretty penny though. My outside of raiding options are Normal/Heroic Oculus, Normal Utgarde Pinacle, Heroic Stratholme, and Heroic Old Kingdom. It will probably be easier finding a group for Heroic Strath than Heroic Old Kingdom.
The allure of getting something right now is certainly strong. Of course as soon as I do, something will drop in Naxx, but I really can't worry about that. I need to go into the raid with the best gear available to me.
1. Buy Libram of Obstruction
2. Buy Plantium Mesh Cloak
3. Enchant Cloak with Titanweave
4. Hope to get an invite to Naxx-25 tonight