Some say that hashrate difference between RX570 4gb and GTX 1070 is not that big. As I said I want the fastest ROI.
GTX 1060 6gb is reasonably priced too.
Also I think 4gb is not big problem because I can also upgrade by the time dag crosses 4gb and there are other coins to mine too. I have seen mixed opinion on this one, some say that before dag crosses 4gb etherium will go pos.
I would always suggest going for nVidia's GTX 1070. It has good hash rate, and it have wider mining possibilities when we talk about different algorithms. I use GTX 1070 for my rig, but RX 570 are also good choice I guess. Personally, I would always go for nVidia GPU's. And yes, I would agree with you that price for GTX 1060 6gb is reasonable.