This is indeed very subjective.
You will see all Mystics saying Mystic is the best choice and same is for Priests.
I played them both but at the end of the day i go on with my Priest, I like it more.
They both have different play styles, so the choice will be yours depending on which one you enjoy more in playing obv. To keep up the Party alive, they are both good. After awakening Priest became far better re. party buffs. Mystic is easier while you solo btw, they have more dps options. Soloing and lvling with Priest is a pain. Same for dodging and iframing, which is easier with Mystic. Mystic is mostly out of combat during dungeon fight (correct me if this has changed) but Priest is constantly in combat mode making your adrenaline up at all times 
Mystic gives you more sustained heals while Priest can burst-heal you and save you in harder situations. Try both for a while and see which one fits better your play style.
