I use both Kindle Fire HDX 8.9 and Paperwhite, and use both everyday. I have owned Kindles since first introduced and, in fact, in our family my daughter has a Kindle Fire 7" and Kindle Fire 8.9 and my wife uses a Paperwhite. We use them all for various purposes. The Paperwhite is essential for heavy reading and for outdoor use, particularly at the beach and is much more comfortable for reading in bed, or for extended periods of reading. It is much easier to hold and much easier on the eyes.
My daughter uses her Kindles at school and for various games and for reading. She is 8 years only, so the color is essential and the 8.9" is better for her illustrated science books. We also have an iPad, but it gets much less use than the Kindles. My daughter is the primary user of the iPad, using it primarily for Apple Apps not otherwise available on Kindle.
I use my HDX regularly for surfing the Web, email, review of bank accounts, reading the NY Times and Washington Post and Politico, and all things a table could be used for, except anything which may involved anything other than very light typing. Obviously we are a committed (perhaps overly so) Kindle family, but to the point of the survey, I think both a Fire, preferably larger screen and a Paperwhite are necessary if one is a heavy reader, which we all are. I also find the Paperwhite preferable for reading the Post and Politico. The navigating for those publications on the Paperwhite format is easier and quicker to use.
One final point, which is also a question and that is comparing the new Kindle 10 with the HDX 8.9. I have considered going to the Kindle 10, but so far remained convinced that the HDX 8.9 is better is all respects, except, of course for a slightly larger screen. I would be particularly interested in any other comments about the comparison, especially whether there is any significant improvement in reading PDF files on a 10 vs. the 8.9.
Use Fire phone to read Audiobooks in bed, Paperwhite for all sight reading except manuals and magazines (iPad for those), Fire tablet is used primarily as a music player on a mini-stereo system in BR. It used to go with me on doctor appt., etc., where I now take the phone. I also have an old Motorola cell phone which is not connected to any phone service, but lives in the bathroom for a quick check of email or to continue listening to a book or music. Used to keep a device in the guest room, but all of our guests seeem to bring their own devices ... or would rather talk to my mistress, Alexa.