The more buffs you can cover the better, however 10 man groups aren't balanced around having all of the buffs. If you can gain a comp that can get them all you'll get a nice advantage.
I will agree with the above poster though. Unless you are in a very very highly rated guild, it doesn't matter what you play. There will be no real advantages of using 1 tank over an over so just play on which you prefer. You'll usually be best at the class you enjoy the most.
Go with whichever tank class you feel more comfortable playing in terms of staying alive. If you haven't played your Guardian since the 5.04 patch, you're in for a big surprise, though. We got reinvented from the ground up, and one of my guild's other bears tank kept doing things the way she was used to in Cata, and she was super squishy. I can't speak to the DK changes, since I've never played one.
On balance, bears are an order of magnitude squishier and require lots more button pushing and split-second decisions based on factors all around you, but once you get the hang of the SD/FR/Mangle dance, it's definitely more of a fight-for-your-life sort of thing.
The most important thing in a tank is understand when and how to react to situations. Whichever one you do that best with is the correct one.
Having said that, if you truly are equal with both and like both well enough to not care which you use, here are a few things to consider:
1. DK can brez more easily. Bear can glyph for 100% brez.
2. DK uses plate and a traditionally dps weapon, bear uses leather witch USED to be uncommonly sought after. With monks and rogue legendaries last patch, I would assume there is a little more competition for leather now but it will depend on your raid composition.
3. DK was the model for tanks this xpac. They have gone through the least changes and are likely to have the least changes done to them during this patch as well.
4. As stated before, buff coverage should be considered if the skill/knowledge with both is truly equal.
5. Both are on the same token so no difference there.
6. What is the other tank? If you are truly equal with both, then a dk and bear combo is usually better than bear/bear for instance.
7. Utility considering the rest of the raid comp. If there are no other dks, amz and dg can be amazing to have in places (Sappers on Blackhorn for example). On some fights, charge and bear stuns may be better utility. All depends on who else in the raid can provide similar utility.