Well, I sum it up this way.
The e-paper based Kindles are supperior devices for reading. The battey lasts longer, and you can easily read them in direct sunlight.
The iPad is solid in-door reading experience. It's main advantage is that you can get content from many more sources (as well as Amazon).
The tablet Kindles combine the weakness of both devices. They have more limited content (book wise) and are harder to read outdoors. But they do give you better access to Amazon's other content and Prime services.
So;
If you want the best ebook reading experience, and don't mind only getting Amazon''s content (which is the largest ebook store), it's pretty hard to beat their cheaper e-paper Kindles.
If you wan't more than ebooks, and/or more content choices; then the iPad is hard to beat.
If you want more than ebooks, but are willing to settle for Amazon's content (or really like their Prime services), then a Kindle tablet will save you some money.
I choose the iPad, but then I'm not just reading ebooks.