I have both the Kindle and iPad and find the iPad to be clearly superior in reading pdf documents. Much easier to adjust the viewing parameters, navigate through large documents and annotate text. Haven't tried the Kindle Fire or an android tablet but I think they also would be superior to the regular Kindle.
The screen size for the Kindle Fire is a little too small for may taste. Also not sure how well the Android pdf apps match up against the iPad apps. The GoodReader app is the best one I have found.
I know there are a lot of claims about how much better the E ink screens are compared to backlit screens, but there is not to my knowledge any scientific evidence to back up those claims. Most of the eyestrain issues related to reading on a computer screen are due to poor ergonomics. My eyes are well past middle-age and I have found reading on the iPad to be as comfortable as on the Kindle.
Why, he's at worst your poet who sings how Greeks
That never were, in Troy which never was,
Did this or the other impossible great thing!