I used Kobo Minis for a while and had several with 'screen fade' issues. It seems to be a manufacturing batch issue; some I had were just fine and others were hopeless from day 1. Others I've spoken to had no problems ever, but I note that none of them were here in NZ (we often seem to get sent the tail end of the quality bell-curve). If you've got one that works, even if it fades intermittently, then stick with it. I have no experience with the Nook.

I ultimately got fed up with replacing Kobos and went for an Onyx Boox - Android e-ink with WiFi and a proper touch screen (so it still keeps working in strong sunlight). They've put some effort into improving image quality and reducing ghosting, and I've never had a problem with it after three seasons.

DH
TX