UD will give you an extra way out of some kind of CC. ( personally I wouldn't consider this since warlocks have UW )
Troll berserking is good for afliction. Orc blood fury is good for any of the specs, and the stun duration is more valuable compared to slow duration, if you are being trained its not like the slow will go away anyways...
Orcs offer a 15% stun reduction, 2% increased pet damage and a 2.2k SP trinket, all bonuses that are very helpful right now. With orc you'll see a noticable increase in burst damage and with the 15% stun reduction you'll be able to get 1 second earlier out of most stuns, which is really helpful nowadays with the amount of stuns going on.
Undead have Will of the Forsaken and Touch of the Grave. WotF is a very nice ability to get out of fears, more or less working as a 2nd PvP trinket against warlocks and priests, however rather useless against classes that don't have a fear ability. TotG is a little boost to sustained damage and healing, gives the best results when multi-dotting.
Trolls give a 20% haste cooldown, 15% reduction on movement imparing effects and generates ~250 hp/sec. Trolls give the strongest burst increase for Affliction, however the 20% haste is of little use for Demonology ( Due spells being instant) and Destruction (Due being easily interrupted during burst). The reduction on movement impairing effects is of little use as well due how easy it is to apply them and the passive health regeneration is just pathetically low.
I'd say that currently orc is by far the best race as you get a nice increase in both burst as survivability. Undead and troll work as well, however slightly less effective.
That being said, unless you are aiming for gladiator, it doesn't matter which race you play. They all work and the differences ain't too big, just play what you like.