Do any of you own both a Z5 and a Z6? How would you compare their image quality? Do you think a Z5 would be a good back up camera to a Z6?
I don't exactly fit your question in that I have a Z6 II but no Z5 or Z6. The Z6 II and Z6 have a backside-illuminated sensor whereas the Z5 has a frontside illuminated sensor. There are some differences reported in the photographic dynamic range across ISO settings; the Z5 and Z6 II have a slightly greater dynamic range at base ISO (100) than the Z6 but the Z6 and Z6 II have a higher dynamic range at ISO 800 and higher than the Z5 (according to
www.photonstophotos.net). Thus the Z6 and Z6 II are probably better low-light cameras whereas for landscape on a tripod either the Z5 or the Z6 II would probably be slightly better. The Z7 II and Z7 would probably be the best Z mount choices for landscape though. I understand that budget can limit the choices and so I would make the choice based on whether you shoot a lot at high ISO, in which case the Z6 would be the better choice of the Z5 and Z6, and if not, then either would work with the Z5 perhaps being slightly better. However, I would also listen to user reports on other aspects of the camera.
For me the Z6 II's value is in its quiet operation (compared to DSLR), excellent high ISO image quality and access to Z lenses. I photograph people often and find the Z6 II's wide-area AF mode with eye/face-detection my go-to mode for autofocus. This mode isn't available on the Z6, unfortunately. Full auto area modes (which also support eye detection) tend to lose the subject more often and I find them more difficult to use. For non-people subjects I would think the differences between the cameras to be fairly small.
Personally I prefer CFexpress type B / XQD cards as I find them more rugged and reliable than SD cards. But I can understand the cost benefits to SD.