Roll a Warlock, simply because they benift each other GREATLY if the Priest is Shadow and the lock is... well... anything really :3 With a Shadow Priest causing mobs to take extra shadow damage, the lock benifits. A Destro lock gets a reduction of shadow resist which benifits Shadow Priests. The Lock pets get a large damage increase (felguard, VW, Felhunter, Succ) with the priests +shadow damage... I could go on, but I digress!
IMO Roll a Lock, they are fun and easy to lvl, you can choose any spec and still be wanted in instances AND it will make lvling togeather alot easier, with your VW tanking whilst you two nuke it :D
P.S. If you do roll a lock, put your first 5 points into Imp. Corruption, it meens you gain alot more manuveability :D