From my personal experience playing both, assuming that you roll similar DD-type r8 on either one, they both can do disgusting damage, as well as lay down great heals for your squad. The only drawback while supporting as a mystic is lacking an aoe heal with as much range as chromatic.
While they both can excel at healing AND damage, a cleric will have a slight support advantage due to buffs and a better aoe heal. A mystic can still do it almost as well, while doing better damage vs most arcane classes. However, this is debatable due to metalmage skills.
The play style is different, but I can say that a cleric can usually support better. Due to channeling, a mystic would have an easier time DDing in most cases. Still, the damage evens out fairly well. There is no right answer unless you have a specific goal/play style you wish to achieve. You can succeed as both.
Touching back on what someone said in a previous post, you can heal and DD as either one in mass pk, but when you're getting focused; the class with a faster heal might serve you a bit better. I don't even enter metalmage mode in large fights when I play DoctorCynix. Way too much focus. A mystic on the other hand maintains access to all of their DD skills while still having an instant heal. In a mass pk environment, it is easier to be a DD mystic than a DD cleric.
Still, I can't give much more insight without knowing what you'd like to do in more detail. Hope that helps.
In my opinion: Damage Supporting
Small scale grouped pk: Metal Mage > Mystic Cleric > Mystic
Large scale Mass Pk: Mystic > Cleric Cleric > Mystic
PvE: Mystic > Cleric Shouldn't matter much
While they can both succeed at both tasks in any of these cases, one tends to have an advantage. However, this does not stop an extraordinary player from proving my opinion to be false.
1v1: They can both destroy any class if played properly, but fighting each other; a mystic tends to win.
