Not an expert here, but i would take the Stamina one with an Agi gem in it (and reforged to Dodge).
But the Agi stone is not bad either (and probably better in terms of pure avoidance/migration), they might be equal.
Best thing you can do is make both and swap whenever you need them.
Not to keen on posting on forums.. but basically its one of those things that you play with as a druid. Basically I base what trinkets and whatnot I wear based on if im raiding or just doing randoms. The one thing you want to pay attention to is you avoidance/mitigation not migration. Were not trying to escape mexico. Personally (I dont know your stats) but as long as your around 33% dodge and around 15 mastery you will be fine as far as avoidance/mitigation goes. Heres a basic rundown on what Agility does for you. 1 Agi = 2 AP while in Cat or Bear Form, 1% Physical Crit per 324.85, 1% Dodge (pre-DR) per 243.58281085. Basically once you hit 33% dodge and whatnot you can basically focus on Stam for a bit because it may not improve your avoidance, but it does improve you mitigation which is something your focused on as a bear tank. Im running 33% dodge 15 mastery and have about 170k hp non buffed.
For those who don't know what the term Mitigation stands for. Mitigation is the effort to reduce loss of life. It speaks for itself as far as im concerned. If you have any questions feel free to talk to me directly or I can post here. Druids are one of those classes that you have to find what works for you and your play style.