I think warlocks are better for PVP than mages (instant cast dots, fear and death coil is great and warlocks are a lot more tanky. Mages kinda need to hardcast spells and face target unless they just spam arcane explosion but that isn't great if you can only hit 1 target...).
Multiple warlock pets are also great for pvp so you can have imp (hp) and felhunter (silence, dispel magic, stealth detection) or possibly succubus for CC.

I think mages are better for PVE than warlocks (about same aoe damage without having to use their hp, threat reduction etc).

I am running druid/priest/mage/2 warlock.

I hate my mage, it makes water/food and can make portals, that is all it is good for... I think I will make a secondary team and replace the mage with a warlock, so I can summon mage when I need its services but use a warlock instead since my mage currently doesn't contribute in pvp fights.