This is my experience. As you can see in my signature its not very much seeing as how I am only level 38 but I will try to explain it.
Imp - Good for higher levels when you can use him as an mana battery since he
can stay phase shifted and never be hit. Stamina boost is always welcome
though at your level it is kinda low. Not a bad choice.
Void - The ONLY reason to have this pet out in BG is for his sac. If you get
jumped and are taking way too much damage his shield might give you the
edge you need to survive. This, however, will burn through a lot of
shards so pack accordingly. Limited use.
Succy - A very strong pet for two reasons: High DPS and her Seduction ability.
CC is always needed to get the edge in PVP and, lets face it, Seduction
rocks. This is my personal pet of choice. Keep an eye on her health
because she cant take a ton of damage. Highly versitile.
Puppy - Good anti-caster pet combine with a great anti-caster class means you
will be the bane of all clothies you see. Decent DPS and Devour Magic
AND Spell Lock make this the favorite of many PVP warlocks. Also, if a
melee class trys to take a whack at it they will be afflicted with a
nice debuff called Tainted Blood. Very useful vs Casters.
Guard - As I dont currently have this pet (being that I am too low) I'll simply
comment on what I have heard. If I relay any faulty info, please correct
me. This is an awesome PVP pet. Intercept is godly with its stun and
he hits like a bus. You must, however, spec appropriately to support him
because he eats through his mana very quickly. With the correct spec
this is probably your best bet for a PVP pet. Very Strong Choice.
I hope I have clarified this information to you. I didnt go through our other two possible pets because I see them as nothing more than fun, novilty pets with limited PVP usage. Again, this information is coming from a level 38 warlock. If a higher level comes in and tells you I'm wrong listen to him. Good luck with your PVP encounters.