I have a Kindle PW and two iPAD Pros 9.7.
The Kindle can only be used for a small number of ebooks that don't rely heavily on images and a formatting that is easier to adapt. A large number of my ebooks can only be read in the iPAD, because if I convert them or even if I scan them again, this can't be done if they are not mostly text.
Besides, it takes a lot of time to scan a book yourself and review the OCR stuff, using ABBYY FineReader. The downside is that the PDF (in case there isn't even an ebook released officially) becomes huge, if consists only of images with a reduced resolution. It can waste 100-300 MB.
I often use this site to convert ePUBs too, for the Kindle format:
http://ebook.online-convert.com/convert-to-mobi
And then I send to the Amazon cloud.
It's always best to search for a Kindle version created by the publisher, instead of doing any sort of conversion. And the Kindle is worthless to read PDFs, most of them can't be read properly and the zoom doesn't work fine with them. The experience is dreadful compared to a file that was created for the Kindle in mind.
As for my experience using the iPAD, it's not ideal, yet it's a good one. I turn the brightness down, enable options such as True Tone/Night Shift and use the FoxIt PDF Reader. Also recommend using a tablet stand such as TStand, or one in the form of a triangle, that holds the iPAD that way:
http://i.imgur.com/vv7DpCu.jpg
And you should be concerned with how the lamps in your bedroom affect your experience. I only use one, a 3000K LED, and it is behind the monitor, according to this:
https://www.howtogeek.com/213464/ho...le-watching-tv-and-gaming-with-bias-lighting/
http://www.neogaf.com/forum/showthread.php?t=1204226
https://www.reddit.com/r/LifeProTip...having_a_light_on_behind_your_monitor_like_a/
This might seem a small advice, but it makes A LOT OF DIFFERENCE if you don't want to ruin things, turn off the lamps overhead, they are too alerting.