It’s actually not a good time to decide.
When the expansion is over, people in the Beta and then the PTR will be explaining what the classes of the next expansion are shaping up to be like. That’s the good time.
It also depends muchly on what type of content you intend to aim at. As we know them, as I see it, each has a small edge depending on the type of content: For questing, DH. For dungeons, Rogue. For PvP, also Rogue. For raids, Warlock. But any one of them will be viable or superior in all areas, so that’s just one consideration, not decisive.
Of the three classes, looked at from the general competitive angle, Rogue is the clear winner for me because at least one spec is always outstanding at something every expansion, but are you really going to be a competitive player? and even if so, that again is one consideration, not decisive, since all three will be viable, and you will always play a class you enjoy better than one you don’t.
People before have touched on this point, but I think it’s the most important one: do you want to play ranged or melee? You say you have experience only with Warlock. Do you find Warlock too slow? Would you prefer to be hitting faster, dodging and weaving in the enemy’s face more? Or are you happy carefully choosing your angle and your shots while your demon, or the tank, keeps the mob occupied?
A lot of people have a strong preference for melee or ranged. General Ranged people go Mage or Warlock, maybe Hunter; general melee people go Rogue, Warrior, may-be DH.
If you are happy with Warlock, then keep it.
If you want to change things up to be faster, more movement, but more vulnerable, then try Rogue. Note I say “try” Rogue, not “go” Rogue. Rogue is its own thing, and you will either get it or you won’t.
If you like the speed and close engagement but find all the Rogue stealth stuff too fiddly, try DH. My reservations about DH have already been expressed: it has only one DPS spec, so if that one gets messed up, you have nothing to fall back on, and it’s due for a rework that may leave it significantly changed from what it is now. The others give you three specs to choose from.