Functionally, all e-ink readers are very similar. Most are touchscreen, so the main choice is backlit or not.
Lighting is well worth while - when I had an unilluminated e-reader (a Sony), I really only used it when I had to, but when I got a Paperwhite, I used it in preference to paper (assuming suitable books).
If you can't cope with tapping on the screen to change page, buttons are still available as well on some more costly models (Kindle Voyage & Oasis).
Practically speaking, it's a choice between Kindle or Kobo since Nook pulled out of the UK,
semi-stranding their users.
If you are likely to take the reader out & about, budget for a cover. A proper cover like the official ones, with a close-fitting shell, and magnets to hold the lid shut and turn it off. The cheap covers with elastic over the corners are rubbish.
Kobo Aura and Kindle Paperwhite are much the same, other than UI differences that you'd just get used to.
Kobo do a waterproof model, and an extra large 7.8" model (vs the normal 6")
Kindle do a model with a battery case that supposedly allows longer away from power sources*, at fairly minimal extra overall weight, though user reports are less enthusiastic.
Kindle do a cheap & basic non-illuminated model
* I find (Paperwhite) that I charge every 3 or 4 books (what would be normal paperbacks).
Kobo won't read Kindle format e-books from Amazon; Kindle won't read ePub books from other sellers.
It's easiest to convert ebooks between formats in Calibre (Windows, MacOS or Linux; no Android version).
The problem is that you can't convert an ebook that's got DRM (i.e. it's encrypted), so you've either got to remove the DRM or restrict yourself to non-DRM ebooks (eg out of copyright from Gutenberg etc, or some more enlightened publishers).
Google is your friend here. I followed apprentice alf's instructions, and now have my Calibre set up to automatically remove the DRM from any Kindle AZW file I import into it. I've found the Kindle store to be much more usable than the other stores I've looked at, so I've never seriously looked at stripping the DRM from Adobe Digital Editions ePubs.
A gotcha is that Amazon have lately introduced a new KFX format that nobody seems to have cracked yet, so rather than letting the Kindle download it over wifi, then connecting the Kindle to the laptop by USB, and adding the books into Calibre from the Kindle, I have to go to "Manage Your Content and Devices" on my Amazon account page, click on the button to the left of the book, select "Download & transfer via USB" (which gives an AZW file), then add to book into Calibre from my download folder.
You don't actually need a working Kindle to do this; a dead Kindle will be OK, so long as it was registered to your Amazon account when it did work. You need the Kindle's serial number to set up the DRM removal, but that's also on "Manage Your Content and Devices".
