Tanks are probably the specs that receive the most balancing tweaks and hotfixes by a large margin. This type of discussions is exactly why that happens. The top guilds typically run with tank players who has one of each (often a backup of each as well). So they will not hesitate to swap to whatever tank is slightly overpowered in their pursuit for world first.
The consequence of this is that the next tier of raiding guilds are often taking cues from the top guilds. So any parity towards a specific tank is quickly rippled down through the playbase and will affect what classes people choose to play. Since Blizzard want as many tanks as possible (as they have always been a limiting factor in casual group forming) they are very quick to react when some tanks are flagged as "bad in this tier".
Single encounters, however, are typically left as they are. Which is why pallies often get to bubble off stacks here and there, or otherwise cheese mechanics. Druids, warriors, DKs, and monks each have their own encounters where they shine. There will always be a little gap between the tanks, but it has historically been <10%. For any guilds other than the top of the pop it makes no difference.
What is far more important when just starting out on a new expansion is to make sure your raid group does not have two of the same tanks. Having two different tanks allows you some flexibility if an encounter design is just gimping a certain class.
I raided as prot MT for many years, from vanilla through 2 expansions. Warriors where in all that time the baseline the other tanks got balanced around. So with one of those you are pretty well off. If you were to pick another one, I would look at your tanking buddy's lineup. Is there a non-warrior main there, I would go for a leather tank. Both druid and monk have trifecta-spec-ability, so they are both great back-up additions for any raid group. Personally I swing towards druid because it also has the choice of ranged/melee dps and the druid healing is beyond awesome for "when you need that extra healer to keep the raid up". Downside of druids is the forms. If you want to flash your gear, druid tanking is not for you. Plus it seems that the ability pruning in WoD is hitting the druids pretty hard, so they might not be fun at all (less buttons and all that).
Personally I have leveled the ones I might want to play to 90 (Warrior, DK, Druid. The pally and monk is just not for me), so that I can spend a couple of days feeling all 3, and then picking which to continue playing.