Lots of good comments, so I will quickly just add my thoughts. I have an iPad, Kindle paperwhite, Google Chromebook and Google Nexus 4. As this suggests, I can't live with just one tool to fit all jobs. When I wish to write, blog or add an extended post, like now I use either my Google Chromebook or my iPad with a bluetooth keyboard. When I wish to read a novel - no pictures or graphics I always use the kindle, it is very light and great in all light condiitions - perfect for trains, tubes, buses , holidays etc.
When I want to read magazines or technical books, I switch to the iPad, and if I'm on the road and nowhere near WIFI, I use my Google Nexus 4, but only for short periods. I'm not sure I would recommend it for long periods of reading, though.
I fully expect to get a Google Nexus 7 when the new ones come out in July, as I like the idea of 4G on the move.
Basically Brian hit the nail on the head early on, define as much as possible the requirements, and if you are only going to buy one device, get the one that fits the most needs. Sheila, I think you are coming to the best conclusion for your needs, and either a Google Nexus 7 or Amazon Kindle HD will do the job. I personally think the Google offers a little more and is not so tied to just one company, as you can read books from lots of people, not just Amazon.
If you want to spend more cash then an iPad mini would be ok, but would not offer anything more really over the other two, except in your case compatibility with your iPhone exeperience.