I have both a Kindle Paperwhite and a tablet, admittedly not a fire. I've tried reading on my tablet, but the light irritates my eyes, and I find I can't read on it for a long period of time. I spend hours everyday on my laptop as it is, so I feel like I need to give my eyes some rest.
I find reading on a Paperwhite so much easier, and more relaxing on my eyes. Plus, if you already have a tablet and enjoy using it, I don't think you really need the Fire. Although I have never used either the Surface Tablet or the Fire, so my opinion comes from no experience whatsoever.
If you are interested in getting a Fire, perhaps try it out in a shop and see what it's like? Maybe it's not as straining on the eyes as other tablets.