The title states it all. i need to find a class i want to play soon. I WANT to play a dps caster, but so does every man and his mother. Should i play something else that is more needed, or should i play a mage, warlock, shadow priest???
If you want something with group flexibility, go Shadow Priest. Even speced Shadow, you can heal your average non heroic 5man, it's just a bit harder since you aren't speced for it.
Play what you want.
all the classes are fun. prepare to waste days of your life finding a class. i had 66 days played on my first mage when i went back to finally finish leveling my priest, then he was my main, then i switched again, and again, eventually rerolling this mage on deathwing.

don't think of it as time wasted, its time playing wow, that is all.
This is something you can only decide for yourself.

As a side note, how long does it take you to get dressed in the morning?
OMG WHAT SOCKS SHOULD I WEAR??????? 
If you roll a SPriest you will be asked to heal nearly every time you join a PuG. Take that into consideration if you don't already have a guild.
As for Mage vs. Warlock.
I would personally go with a mage, I find them to be more entertaining than playing a lock.
Everything is wanted in 5 man groups, even heroics... and guilds are looking for almost every class/spec as almost every of them have some sort of utility (tanking, dpsing, healing, support... etc.)

If you want a caster, chose whatever caster class appeals you the most.

- Druids are versatile casters and melee (can do ranged dps, healing, melee dps and tanking) depending on their talents and gear.
- Priests can heal, protect and dps depending on their talents.
- Mages can only dps with massive burst damage.
- Warlock can only dps, have a pet with some utility.
Play what sounds fun to you, if you pick a class because you think other people will like it, or because someone else tells you to pick it, you will probably be unhappy.

If you want dps casters that still can offer needed group utility, priests and druids (Boomkin=caster) can dps, and can spec for the much sought after healing.
You don't need 2 threads for this. play your priest. We command you.
You don't have to choose. Try a class, keep playing it if it's fun. Try another class, same deal. Also, don't ever delete a class, so you don't have to start over if you get bored with it, but want to try it again later.

You've got 10 slots to spend.
Play the class you want to play. Generally when you play a class that you enjoy, you will strive to be better at that class and therefore be more valuable to groups. You may not be the most useful but if you work at it and strive to be good I doubt you'll have trouble finding groups. 

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