Here is my take on the eReaders currently available.
Sony ereaders. Love them. Software is relativelyeasy to use and make purchases. If you have a complete reinstall, you can re-download your purchased books for no cost. This is the one I used to use, and the sony's got me thru 22 months in Iraq. Extremely long battery life. Downside: Most of them don't do wi-fi, nor have backlight or any kind of lighting scheme to make it easier to read in low light. Bring your own flashlight.
Kindle. I avoided them because they have in the past gone into people's accounts and removed books. Granted, they DID issue a refund for the book. However, the endusers were not ASKED, the books were simply ... gone. Doesn't matter why. So I like my book accounts disconnected from everyone who might try to control what I can and cannot read.
Apple. The ipad 3 is decent, and not overly heavy for e-reading. Mind you, I avoid Apple's proprietary ebook software. The good news is the IOS platform uses 3rd party software too. So if you already have a library under Nook, or Kindle, or Sony ... they all can be read on the apple device. I am now using my Ipad Mini as my book reader. It's awesome. Excellent form factor, battery life while reading AND listening to music is excellent, and the screen is of course backlit.
Recommendation: If you're going to have an IOS platform OR an Android tablet, the stay with it. They can use the 3rd party software and you save money and don't have an extra device laying around. (My tablet that my wife uses is a Samsung Galaxy Tab (v1) 7.7 (7 inch screen... she loves it!)