I can attest that the Kindle Fire is a perfect reading device. Perfect form factor, light yet feels great, etc. I really don't think an iPad is great for reading based on size, but I've never had one long enough to try it.
I had a Fire and really didn't like it for reading -- it falls in the valley of uselessness between a Kindle (DX being my favorite) and the iPad. I don't find my iPad 1 particularly heavy -- the main drawback of the iPad vs. Kindles is lack of daylight readability, and to a lesser degree, potentially distracting apps (email, web).
The Fire seems like a great video watching device, but I don't do much of that (and when I do, it's on a projector with surround sound and other people)
Quite on point. Few people would willingly admit to this, but a lot of research suggests that simply having options available imposes a real cognitive load. Simple additions to text like footnotes and hyperlinks produce real drops in recall/comprehension - it's not a stretch to apply this to the iPad with its notifications and rich entertainment options.
The kindle has only two things that make it competitive enough to be worthy of discussion- e-ink and lighter weight. In every other category (ok, excluding price) it fails compared to the iPad.
on e-ink there isn't really an advantage to my eyes. I've never gotten eyestrain reading on my (first generation) iPad. The image quality of the e-ink display is worse than even the first generation iPad (to my eyes), in large part because the e-ink display is not responsive.
weight- I deal with it on the iPad. The kindle is better in this regard, but not enough to make it compelling.
Amazon had a great product in the original kindle, but it was a single purpose product. It hasn't kept up (or e-ink hasn't advanced fast enough) and its losing the race.