I've played both classes to some acclaim and as far as enjoyment I beg you to consider playing a Priest.
No trolling here. I'm just someone that also gets gung'ho about using magic to destroy my enemies from range which I'm guessing is a playstyle you prefer as well. I will say that I played a lot of Mage and Warlock and it was a lot of fun, but now my Priest is my main and I run as a healer. Playing a healing class I found is a much more rewarding experience than competing in a DPS race. If you still want to do some dps you can always go Shadow as well. As a Priest you're still a clothie spellcaster with an arsenal of spells, the only difference is a lot of them are for defensive use
I'm not telling you to play a Priest. I'm just saying from my personal experience, I wish I played one much sooner. I hope you enjoy this game no matter what class you pick.