Draenai's passive scales to 4 in Battlegrounds, which makes it the equivalent of Dwarf (5 vs 4 + 1).
You also get access to a very strong self-heal, which you will probably end up valuing more than the reduced damage from Stoneskin.
I.E., you are in defensive stance, which means you are only taking 80% damage. Stoneskin reduces all damage taken by 10% for its duration, but Damage Reduction bonuses in WoW are not additive (think 20% + 10% = 30% damage reduction), instead they are multiplicative (20% off of 100% equals 80%, 10% off of 80% = 72%). This means that Stoneskin is slightly less valuable for Warriors than it is for other specs, at least at free to play.
That being said, the advantages of removing the myriad of debuffs are innumerable, and realistically offers you the best odds in any given scenario. I only consider Gift of the Naaru to be a stronger buff because the self-healing is so strong and so needed for warriors.
If you will have a pocket healer, go Dwarf. If you expect that you will not, go Draenai.