if you want self sufficient survivability you're gonna wanna go priest regardless of what spec you want to play, priests can simply just survive longer than warlocks can
you'll probably have more fun playing warlock though as they are quite good, especially if you run with premades. theres a lot of healers so you'll just be another needle in the haystack, however you can fill a more niche role as a warlock in premade groups. not to say that this niche isn't valuable, in fact I love having warlocks in my grps.
but ya back to my first statement, if you're solo qing and want survivability priest all the way, if you queue with the homies you'll be able to survive and thrive as a warlock
or solo q warlock fuck it +respect