I have both the new Kindle Paperwhite and the old Kobo mini e-reader, and I like the Kobo much better. Kobo's UI is far superior, and it connects with Pocket, so you can catch up on your articles too.
They have hidden internal MicroSD cards. A few screws and you can upgrade your device pretty easily.
I disliked the "activation" process of setting up an account stuff and needing to connect to the Internet, but there are workarounds for that. http://uscoffings.net/clc/tech/embedded/kobo-touch/