Why not both? Then you have a melee and caster.
Warlock is the true choice! My warlock was my first character during TBC, and I'm still playing him. Never been disappointed or bored with my choice. I have tried death knight too, it was pretty cool. But not as cool as warlock.
Death Knights are more likely to be consistent and have less large changes into the expansion. They need better secondary stat scaling but their mechanics work.
Warlocks however are unlikely to keep their current strength going into the expansion. Demonology and Destruction are largely ok but we'll likely see Malefic Grasp removed/reworked and significant changes to Warlock utility.
So without knowing what exactly gets changed, if you like the way Death Knights play now, you'll likely like how they are in WoD. Warlocks are entirely up in the air.
Can't decide either
lock is my current main, green fire(so awesome) decently geared(560) so much fun, my first toon made, and the toon i did heroic raiding in Wotlk with.
Dk (blood) decided what the hell, leveled, did flexes, so much fun, solo lk heroic 25? sold.
torn between the two, as much as i would love an answer now, not enough information until late beta.
Wild card: Dreanei ret pally. main toon in BC and towards the end of wotlk, what is WoD for alliance if not a Xpac for Dreanies
If you're interested in doing insane DPS warlock is the no-brainer choice as they are consistently in the top 3 highest dps. DKs will probably be super competitive early in the expansion and then tail off like usual towards the middle and end (like every expansion).
Warlocks (destruction atleast) will get a nerf in WoD (or maybe before). They are the most OP class right now. Warlock is my main and it kind of sucks but im realistic here. That's why i play with all my lvl 90's right now. You never know which one you will enjoy the most in WoD.
Warlocks are almost always a good class to go for Affliction at the start normally pwns everything followed by demo mid expan and destro towards the end or a mix of all 3. DK's normally rule at the start and get worse at the end.
But we can solo more stuff, and we get punched in the face and stay alive for more.
Roll whatever you enjoy, but if you want something to top the charts then i would say a warlock would be higher DPS on average for the whole of WoD. Plus Warlocks always get cool gear and toys.
If you want a stable class roll a priest or mage those guys never seam to get a buff/nerf (same goes with rogues mostly also.)
Go Warlock. Forget the fact that I highley doubt DKs will see any fix in WOD, melee in general will continue to be less desired than range. Go Warlock.
If you want to PVP I would suggest DK
If ranged PVE is your bag, consider a lock.
Lock usually becomes op in later patches each expansion. I'd go for a DK.
Right now I heavily lean toward my warlock. I play both right now. The warlock is destro (557) and the DK is frost (550). I have the legendary cloak on both.
As others have said who knows what WoD will bring... but after playing 4 expansions with one and 3 with the other here are my thoughts this expac:
Leveling was both faster and easier this expac on the DK. I always level as blood.
Fresh at 90 the DK was much less dependent on gear. The lock was a bit painful until I got raid level gear.
Both can solo old content but the DK obviously shines.
Both are fun in pvp.
Current tier I feel like the DK is weak by comparison to every other DPS class ilvl for ilvl. I can pull good numbers with the DK but I have to really work for it. If I land on top of the meter I feel like I accomplished something. It is possible... but it sure isn't easy for me. If I don't find myself fighting for first on the lock I know it is because I screwed up or have less gear.
Love my dk for in your face melee with spells.
go warlock.
there is no point in playing a melee at all and blizz designs the game more and more biased towards the ranged dps