Races and their perks:
Human:- Mace Specialization: Completely unusable.
- Sword Specialization: Pretty damn useless.
- The Human Spirit: Also pretty damn useless. Warlocks do not use Spirit to regen.
- Diplomacy: Amazing for shitty rep grinds. That's a lot of time saved.
- Perception: The most powerful boost to Rogue detection in the game, except it's an activated ability so you need to guess when the invisible rogue is near you before they get the drop on you (also, it doesn't let you spot their special Vanish - stealth level too high), plus you are playing alliance so odds are it's an undead rogue so you just get to acknowledge your impending demise. If you have engineering items this can actually let you get the drop on them though. Great for duels. Good in arenas. Might do something in battlegrounds. Crap for world PvP.
Gnome:- Arcane Resistance: Free resist is always nice.
- Engineering Specialization: If you're an engineer, it's good. And if you're alliance (and PvPing), you should considering engineer because it's a source of crowd control that doesn't have to deal with WotF or trinket. Plus it's a damn useful profession in general.
- Escape Artist: No complaining here. Can be quite useful depending on the circumstances.
- Expansive Mind: Warlocks don't really gear int (Stamina is your mana pool too), but 5% more mana can still save a lifetap, so it's decent.
Orc:- Axe Specialization: Completely unusable.
- Blood Rage: Rather useless.
- Command: 5% extra pet damage is decent.
- Hardiness: 25% chance of ignoring stuns is great since it means a Rogue can actually fail at stunlocking you to death.
Undead:- Cannibalize: Great. It's a heal you can use during combat and Life Tap means that also gives you mana to keep going if you ever have one of those annoying drawn-out combats with a tank.
- Shadow Resistance: Free resist is always useful.
- Underwater Breathing: Pretty useless, since Warlocks get Unending Breath at level 16.
- Will of the Forsaken: Overrated on Warlocks, since your trinket already breaks this, and if you have a Felhunter he can eat the CC off of you, and if you have a Succubus you can trade CC too. It is worth noting however that the opposite is also true: With Will of the Forsaken you won't have much need for a PvP trinket. Still a strong racial at any rate, just not as amazing for Warlocks.
First things first:
Horde faction is best for PvP. This is because of Will of the Forsaken. While it's overrated on a Warlock, it's fucking ridiculous to go up against as a Warlock since you have no escape abilities other than "CC someone and run away", and the Warlock's CC is Fear and Charm (Succubus Seduce). You're screwed against Undead, especially when you add in a PvP trinket for even more CC breaking plus your Diminishing Returns. Also Undead have racial shadow resistance and odds are 90% that is how you do your damage. So roll Horde so you never have to PvP undead.
Second things second:
Alliance faction is best for PvE. This is
also because of Will of the Forsaken. Getting Warlock gear has never been easier when there's half as many Warlocks (or less!) in your raid. You also get a faction benefit in the form of the Bag of Marbles quest reward (BoP, 10 charge Consumable: debuffs target with +25% miss chance for 10 seconds, 1 min CD, use wisely) which can make a great ticket into a raid if you're under-geared if you promise to use a charge on the raid boss. Plus there's the Light of Elune quest reward which gives you a 10 second invulnerability shield for a minute (1 charge only, BoP). Fun fact: Both of these used to be BoE but people rolled a ton of alts to farm the shit out of these items at which point Blizzard changed it to BoP. Aside from that, Alliance locks can benefit from a Paladin's Concentration Aura which stacks with your talent tree to make Drain Life/Mana/Soul, Rain of Fire, and Hellfire immune to casting pushback.
For specific races, PvP-wise, on the Horde side, Undead is better against other Warlocks and Priests, but those are matchups you can handle anyway. Orc on the other hand will give you a stronger chance against Rogues since it messes up their stunlock, the Warrior with his Charge/Intercept, Paladin stuns, and anyone who uses engineering grenades. I would favor Orcs strictly for Hardiness, honestly, since it improves some of your worse match-ups. On the alliance side, Gnomes are probably better for PvP given their Escape Artist ability, arcane resist, etc. PvE-wise, Undead make better Warlocks for Horde given their Cannibalize, and Humans make better Warlocks for Alliance given their superior rep grinds.