What device do you read on? If it is your iPhone then Kindle's app is far superior. Apple's iOS and iPad Book App will not take advantage of the entire screen - forcing massive margins on the side while Kindle's app can utilize the full screen space of an iPad/iPhone. Mac OS this is not an issue.
I started out on Google Books, went to Kindle and years ago (2017?) I went to Apple Books. Apple's book app is not great, and despite requests from this poster for years, they refuse to use full margin space on their own devices. But, it's not Amazon. There's been a few times that Amazon will have the book and Apple does not - and for the most part they're the same price, but Amazon has rewards for buying Kindle books. But for the most part, when there's a sale on Amazon it also is the same price on Apple Books.
Big plus about Apple Books is you can take an ePub file or pdf and drop it into the Books app and have it instantly available on all your devices. I have hundreds of DRM free books in my library synced across all my Apple devices - and this is pretty cool for me.
Been using Apple Books for years and years. If you're trying not to give Amazon your $, go Apple. If you already have a large Apple Books library - Apple Books is pretty good. However, if you answer no to the previous questions - Kindle is far superior. Bookerly is a great reading font. Amazon's Kindle apps actually use the entire iPhone, iPad, Mac OS screen. Kindle apps do a much better job of syncing and remembering last page location. Amazon offers rewards so you'll save a lot more money buying books long term on Amazon. Amazon eBook apps are platform independent too. Also, if you ever wanted to dive into an E-Ink reader device, actual Kindles are amazing.
I'd stay Kindle if I was you.