I wouldn't worry about the case, either way you should buy a cover to keep your device in --much safer.
Amazon has a reputation for rock-solid firmware, far more so than any other ereader. Then again, this may be because they don't offer tons of customizations like Kobo, they keep the whole experience much more stable. I suppose it depends on what you prefer.
Amazon of course has the best store, and if you've bought many pbooks from them, you may want to take a serious look at Kindle Matchbook:
https://www.amazon.com/gp/digital/ep-landing-page
If you don't mind going to some lengths to manage all your books on your computer, using
calibre, then you can use all of these books on a Kobo too (it's really quite easy once you get the hang of it).