Well right now my main team is 1 priest 2 warlock,

and ill tell ya it works really nicely! REAL nicely!.

soulstone, healthstones, tank, healer, mind flay (shadow), dots galore, renews etc etc. Its nice,

So actually a mix is my preferrance, get the best of both classes and they are a great pve mix, being that shadow priest gives shadow weaving to the warlock and the warlock gives tank, great dps, just about everything else in the game.

Dont be afraid to mix and have some fun

IF your stuck on just 2 of the same class one or the other, My opinion says go warlock,

Warlocks in the leveling meantime get 2 tanks, dots, soulstones, good stuff,

in the long run, Demon form... and haunt, omg... omg...