I like the question, as I was asking myseld the same.
Regarding turkish yoghurt I think it's basically the same + cheaper if you buy them from turkish stores. I'm sometimes a little bit unsure regarding the ingedrients as the labels are sometimes wrong when imported. I actually find "greek yoghurt" very rarely only "yoghurt greek style" which is due to law issues in germany I guess.
I was wondering where the difference is between "greek yoghurt" and full fat yoghurt with added cream? In some cases the greek one is strained, but otherwise than that I don't see any difference.
I will probably stick to buying ecological full fat yoghurt as it tastes better in my opion, that non eco "yoghurt greek style". And adding cream isn't that hard in any case.
Regarding "quark" I'm not 100% sure whether it's that good... (it's definitely not bad). But one good reason to eat yoghurt are the living cultures in it. I think here it depends on the production method whether there are still many in the quark or not. One big plus for the yoghurt is, that you simply add some bacteria and don't do much afterwards so that all the nutrients that are in the milk are still there while the bacteria transform some of the whey into protein. For quark you seperate whey from casein by lactid acid which might come from bacteria or rennet, so I think there is some loss here.
By the way, google tells me that the translation for "quark" should be food curd. Also I'm not 100% regarding the information above, maybe there is some dairy expert around here to clarify things.