I don't own the Fire 7 (but I came close to grabbing one at the sale price...)
This youtube video compares the PW3 to the Fire 7 with reading comparisons:
https://www.youtube.com/watch?v=4HqH5VB1s34
That should give you a good idea of what the reading would be like
I also found a written comparison review:
http://blog.the-ebook-reader.com/201...-review-video/