I read on a mix of my phone (if the train is too packed to get to my other stuff) using Kindle app, Kobo app and Moon Reader), Nook Simple Touch (no longer made, but an eInk screened book reader, to which I sideload ePubs using Calibre) and Kindle Fire (using the Kindle app).
(I read paper books as well!)
The Nook is the best reading experience, but like other dedicated e-Readers, it doesn't do much else. (I rooted it a while back, and stuck a compact version of Android OS on it - I could check email &c on it, but have to admit that I don't). I usually get at least a month between charges.
My phone is ok, but has a smaller screen, and is very prone to interrupting your reading! Having the screen on clobbers battery life, of course. As it's a Samsung, I get a free book each month (from a selection of four) through the Kindle for Samsung app. Those are generally a bit ropey, but there have been some good ones.
The Kindle Fire is ok for reading on (7hrs or so of battery life), allows you to borrow one book each month from the "Kindle Lending Library" (other Kindles, including the eReaders allow this though) and is an ok small tablet. If you have a prime account, it will play Amazon video & Amazon music (you can download content from both for when you're away from wifi) and the appstore (although limited) does have Eurosport Player, Netflix, Iplayer, ITV Hub, My5 & All4). It won't Chromecast, because Amazon and Google have had a falling out.
If you only want to read, a dedicated eReader is the best bet by far. If you fancy tablet features as well (watching films, internet browing &c) the Fire is an ok small tablet - underpowered compared to pricier options, but there's little else available for £50 that's as capable.