It's worth noting that you only have to reformat your books once, and you can always leave that job running overnight. Or calibre will auto-convert to a supported format if none of the device-supported formats are available. Don't reject the Kindle as a viable option just because of a one-time formatting need. And the Kindles are known for having
very stable firmware.
Not having a Kobo, I can only say what I've heard on MobileRead, but the key seems to be finding a firmware that you like which doesn't have any bugs that affect your usage, and then stick with that firmware.
Do not get a Nook; in addition to them being on the way out of business, they have few features and crash all the time. They are buggy, slow and obsolete. Several friends of mine have Nooks, so I know.
