Book formats - with the exception of library books, etc that are protected by DRM (Digital Rights Mnagement) - are trivial. I use Calibre to convert my books from epub to mobi (Kindle format) because I used to read on a Sony but have had a Kindle paperwhite for 2 weeks. Calibre will convert pretty near any format to any format and it's as easy or complex as you choose to make it. Simpler conversions or full on changes to the books.
http://calibre-ebook.com/
There are really only 3 things to consider with e-Readers:
1. Do you want it to do everything or be designed for the reading experience? If everything, go tablet. If the reading experience - go eReader.
2. Do you want to borrow library books or would you buy/download/side load them? There are tons of really interesting indie sites out there for books too - smashwords, Baen, etc. If you want to borrow library books - in non-US countries, that's the deal killer for the Kindle. I get around it with my Sony reader but I really don't borrow a lot anymore.
3. Do you get along with how the reader works? Is it intuitive to you, do you like how it operates? Do you like the backlight or not? etc. I find the backlight on the KPW is bluish and glares a little even at 6 out of 24 at night but I'm getting used to it and it's still better than the hotspot glares on my Sony.