I've had a Kindle Oasis (the non-ad-supported version) for a couple years. A couple months ago I got a Kobo Sage.
Same in both Oasis and Sage: - Screen quality - Battery life (in my experience; this doesn't match the actual specs of the products though) - "Waterproof"
Differences: Favoring Kobo Sage: - Screen is a bit bigger without a significant penalty in size and weight - Compatible with a pen and has built-in drawing/OCR/note-taking capabilities - Really easy to borrow eBooks from library via OverDrive - Easier to side-load and sync free eBooks - Slightly cheaper (US)
Favoring Kindle Oasis: - Slightly lighter weight and more compact - More responsive buttons (touch screen is same responsiveness) - Amazon's book purchasing and browsing features are a bit easier to use - Amazon selection is better - "Family" sharing - Built-in free cellular wireless data for Amazon downloads and browsing
I'm happy owning both devices. If I had to choose one or the other, then:
eReader only: I'd choose Kobo only because library borrowing is so much easier. If my local library didn't have OverDrive, then I would choose the Kindle + Amazon Unlimited in a heartbeat.
eInk tablet: Kobo, no question. The Kindle is an eReader. Amazon hasn't made any effort to make it anything more. It simply doesn't make sense to me to compare any eInk Kindle to the Kobo, MatePad, ReMarkable, Boox, etc.
My partner likes eInk tablets for note taking and diagramming, not for e-reading. He has both a ReMarkable 2 and a Boox. The ReMarkable is fine, but He much prefers the Boox because it's easier to use without a subscription. But, now that my partner has used my Kobo Sage, he is unsure whether he'd buy a Boox over a Sage. He says it's a very close call and it would probably come down to price.
A product that's subsidized will always win the price war with one that's not
It's comfort and convenience subsidized through suffering
Not necessary. AFAIK, it is very often the case they just force vendors to not offer lower prices elsewhere, or else face non-promotion on Amazon, reducing sales significantly.