First and foremost, what do YOU want to DO on the tablet? You said you want to use the tablet on the treadmill, I use mine when on our Torture Cycle and I see plenty of people at the gym using tablets. Do you want to browse web pages, play games, read book or magazines?
I have a first edition Kindle Fire and for XMAS I bought the latest Kindle with the largest screen. Oh my but the difference his HUGE between the two. The newest one has a larger screen which helps when reading yet it has more memory is thinner and weighs less than the first edition. When comparing tablets I would strongly suggest looking at weight. I really does make a difference when using the tablet.
The first Kindle I bought was to get the WSJ delivered electronically no matter where I was and to save money. The electronic version of the WSJ was much cheaper than the paper version, and in one year, the difference paid for the first Kindle. Amazon has me hooked. The ability to buy a book and have it within a minute is magic. I have a house full of physical books and I resisted eBooks because I want the physical book but the ability to enlarge photos, tables, graphs in the Kindle books, along with the note taking and highlighting ability has changed my opinion so that know I prefer eBooks. Plus, I can access my books on my cell phone or from other computers. The access on the cell phone is so danged handy. This is not something I thought was use full until I needed the books away from home as a reference or to reread when I had some time.
The latest Kindle is a very good web browser. The old version was a bit slow but the new Kindle is faster and the display is much, much better. Not really sure how it could get much better but I am sure in a few years there will be a "better" display. I don't play games except solitaire and I only use a couple of applications so application choice is not important to me at all. But if you use different apps then you should check the various app stores for your needs.
I don't think there is really that much of a difference in the technology of one brand of tablet to another. One will have an edge today but the same model will be behind the curve tomorrow. I bought a Kindle because of the WSJ and we order so much from Amazon. For us, it is an advantage to use the Amazon ecosystem. Others want to be in Apples ecosystem or none at all.
Later,
Dan