AWS is more comprehensive than both by quite a wide margin. MS does indeed have a long history of relying very heavily on marketing. Time will tell if marketing can paper over the large gap in features.
It would be more useful in a format which works on Kindle, since that's by far the most common. It wouldn't hurt to support both and also pdf. If there was a single page view of the book it would be quite trivial to export it to a single ebook file.