I say why not both? Leveling will be much less tedious in Cata, and DK's start more than halfway there in any case.
Leveling up a new lock could be a real fun challenge.
I'd like to do both, but with work/univ, I don't have the time.
And to Primigenia, thanks for your post. I don't really care about the Pet aspect, just seems like 'something', not rolling either class because they have pets. From reading up on DKs, yeah Unholy seems like the "go-to" spec for pvp/pve, which is fine with me.
Coming from a rogue, I do like melee more. However, all things considered, it's all about next season, and then long term after that. They both seem pretty solid I guess in arenas and raids, but I guess maybe locks coming out a bit ahead?
DKs can roll with warriors/locks/hunters in 3s (off the top of my head for r1 comps), while locks seem like they can roll with just about every class, no? One of my friends is guaranteed playing a mage, with an alt warrior. So would a DK be the best for doing 3s with him, or a lock?