Just go for whichever looks best for you, as long as it supports the EPUB file format, which is by far the most widespread ebook format nowadays. Be warned that the Kindle is particularly bad in this respect, because it only offers partial support for EPUB. You have to go through a whole spiel to get an epub file to your Kindle, whereas with most other e-readers, it's just a matter of copying a file over to your device.

Here's a comparison table for e-readers on Wikipedia. It may not be comprehensive, but it should give you an idea of what functions and/or features you're looking for:

https://en.wikipedia.org/wiki/Comparison_of_e-readers