Not because there are huge differences in tanking ability (though there is in style) but there isn't a lot of need for tanks in 25 mans. As a warrior, if you aren't tanking, you are DPS--and there are billions of DPS around. Healers on the other hand, are almost always needed.
So if you decide to that you really want to see 25 man content, the fall back position of healer or DPS is far stronger than warrior's of DPS only.
Several debates on this already in tankspot's history. Okay, so maybe there hasn't been one for patch 3.9.
The best answer would be for you to try each and see what you like. It's going to eventually come down to what you personally enjoy more out of the four classes. Obviously, that's a lot of work and you'd have to really get each geared to see it's potential. I've played 3 of the 4 now at end game, they are all enjoyable. Probably the most "fun" in ordinary mundane situations like farming was the druid to be honest.
If there are any wide discrepancies between any of the classes you can be certain that there will be a flurry of posts to blizzard to let them know they're doing it wrong. Don't go by who is in 1st place on any particular aspect of tanking, it will change as we play marry-go-round with nerfs and buffs to get everyone "equal". Bank on it.
Originally Posted by Kalken
"I'll let the dragon hit me in the face, you stab it in the ass."