Look for an ereader that has features you like, and don't worry about the format.
calibre will make format something you don't have to think about.
Kindle and Kobo are both good choices, the usual difference is the Kobo has lots of customization options for fine-tuning (nitpicking
) the reading experience, whereas the Kindle has very stable firmware.
Fringe benefits:
You can email books to your Kindle and get annotations synced to your smartphone/tablet.
Kobo has direct integration with pocket.
If you plan on running custom hacks, get a Kobo -- the Kindle is no longer able to be jailbroken.
