Source: Wowhead's level 30 beta guide (cap 30). Level 60 version: Not yet published, updated at launch.
Pet
Quest level
Use it when
Voidwalker
10
Mobs hit hard or gear is weak
Succubus / Incubus
20
Default damage pet
Felhunter
30
Not covered in Wowhead's level 30 guide
Which pet
The Succubus does the most damage of any pet in the beta, about 1.5× the Voidwalker. Without Demonology talents she can't hold threat, so expect the mob on you about half the fight while Drain Life heals you.
The Voidwalker holds threat better than in Classic. Torment gains a rank every 10 levels, and each rank's threat grows for 5 levels then stays flat. From 25 to 30 he loses mobs more often until Torment rank 3 at 30.
