Can't decide which to go for. It's for solo PvE.

For solo PvE warlock, without doubt. Cloth is never good for tanking, Being able to summon something to tank for you is very useful.

Shadow Priests are Awesome due to the fact you can heal if you need to.

I personally had all classes at 80 then quit when cataclysm came out, so I really don't know how they are now.

But PVP Wise of the two Warlock was my fav.

PVE It had to be SPriest

what do u mean solo pve ? you can solo any dungeon or raid !
anyway , if u want dps then choose warlock . they are way too powerful and do massive dps . if u want to heal sometimes and dps sometimes , then go for priest .

I have both and both are good in their own ways.

Priests are good as you can take a healing spec as an dual spec from 30 and this will enable you to get into dungeons quicker and thus level faster. As a S-Priest, the majority of mobs will die just as they reach you when you apply all your dots and mind flay. for multiple mob groups, chuck on a shield before the pull and your good to go.

Warlocks are good in that the pet takes most of the damage, once you get the voidwalker, this is so much easier. Depending on what spec you pick is how fast you will level in my opinon. Demonology is very fast as the Felguard holds aggro very well, and you can pull more mobs while he finishes off his target, and so on, to create chain pulling. Affliction is good for doing lots of single target damage, those Dots REALLY start to hurt and Destro is good for burst damage.

Out of the two, personally I would pick the Spriest if i had to level one or the other. Plus, in recent tests, SPriests are currently topping the dps charts for all current raid content.