I use whatever I have to hand, quark needs a lot of seasoning (IMO!) and gives a smoother texture than cottage cheese but both are good. I've also used the smooth cottage cheese which gives a similar texture to quark.
I'd say use whichever's "use by" date is closest!
I havent made a quiche out of Quark before, so will try that. I like Cottage cheese with chives and onion when I make a quiche, but I find both of them needs a lot of flavouring.
No help I know, just rambling...lol...sorry!
I have used cottage cheese and quark but I prefer quark as it's smoother.
I beat it really well with the eggs - I use a tub of quark with six eggs, and used like this the quark has never "split" for me.
I do season quite well with salt, pepper and mixed herbs.
I use any veg on hand, mushrooms, tomatoes and bacon or ham.
It really tastes yum and I can never make (or eat) enough!!
I usually use quark and it's lovely, but have a cottage cheese one in the oven right now, as an experiment. I bought CC specifically to try it to see the difference. Will report my findings after my dinner/tea/supper *delete as appropriate.