What makes the Kindle (and most other e-readers) better than an iPad is that it uses an e-Ink display so it is very easy on the eyes, like reading a paper page. It's great, I read on mine a lot.
The Kindle battery lasts for a week or more without recharging! That's with the WiFi turned off. I have seen an iPad and it's really very nice, but I think it would be tiring to read on it for very long, and of course the battery won't last as long. Besides, I just want to read books, don't care about all the iPad stuff, and the Kindle is smaller and easy to carry around, and turns on instantly.
For the Kindle, ebooks need to be in either Kindle(MOBI) format or PDF format-- it does not do EPUB. With the Kindle format, you can set any font size you like and it reflows the text to show whatever amount fits on the screen for your font size. The problem with PDF is it displays each whole page as formatted by the PDF file's page size, so the font may be very small. You can zoom in on it, but then you have to pan around to see the page. I have several PDF Dhamma books on it and I can read them OK, but if you have trouble reading small print it may be a problem. I keep meaning to check into whether I can resize the PDF pages/font in a PDF editor or convert them to MOBI but haven't tried it yet. I think it depends on each PDF file and how it was set up.
Also there's a free program, Calibre, that is nice for managing your ebook library and converting between formats.