From what I've read here on TankSpot, all the tanks are relatively equal. Shieldblock from Warriors and Paladins makes more of a difference when tanking multiple mobs with smaller hits.
I think it really boils down to which playstyle you prefer. Take a look at their skills and see which appeal to you more. Personally, I really like the Warrior mechanics, I played one back Pre-TBC and really enjoyed it then. The only reason I play a DK now is he was already level 80 when I decided I wanted to tank :P
Bear tanks will have a higher HP pool than a Warrior will with similar gear. Warriors have shield block/critical block though.
Not sure if offspecs are going to play any part, but Druid can play all three roles, tank/dps/healer, where the Warrior can only fill two.
Both are relatively easy to level (not that leveling is difficult with any class), with the edge going to the Druid I believe as Feral.
If the other tank you raid with is a Paladin, it's it's likely to stay that way, you could go Druid that way you're not competing for tank weapons/shields/plate etc, I'm sure they'd appreciate that
In the end, it just boils down to which playstyle appeals to you more.
Hope this helps.