Priest are better overall in the longrun for everything including cash. People love to pay priest to run them through an instance, raids are only sucessful with priest, and you will never need to wait for an invite to a team lol
Priests are very good at PvP if specd right. Warlocks are natrualy good at PvP. In end game, Priets are expected to spam heals. In the end, it all depends on your play style.
prists are really good because ull get parties for instances better. on the other hand, warlocks are really strong. both of them can only wear cloth armor
but seriously, pallies are the best because they NEVER die. eventually they will win against anything, unless they die in 1 hit. a level 20 pally can kill a level 25 elite by doing this (attack, bubble, heal, attack, bubble, heal etc). they just NEVER die. they can heal and they can attack powerfully and they can wear mail =D
warlocks are kool and fun but if u are a good priest(healer) u could be in any group or raid and prolly get really good stuff then make ur alt (lock) then he got really good stuff then evrybdy is happy