they both have DoTs

Whats the difference between these 2 classes and which is better?

warlocks have minions, the DoT's do different things, as do all their spells. shadow priests do also have a few healing abilities that they can bust out in a pinch. obviously don't count on shadow spec to be able to heal you through a dungeon, but a shadow priest has saved my life on a few occasions when the actual healer got feared.

so while they are both casters, the play style is different between them. take into consideration different cooldowns and such.

the better class is whichever you like to play more. i have met excellent warlocks and shitty warlocks. i have met awesome shadow priests and terrible ones. there is no "best class" despite whatever smoke someone tries to blow up your ass.