@Catsnkites,
Addind sideloaded books to any ereader is pretty easy, there are only two issues: 1) DRM, and 2) format (for the Kindle).
As one of the initiated Mobile Readers (

) you don't have either of those problems.
The exception is of course the Nook -- and only in the sense that the recent models have practically no (externally visible) storage, thus making books in general, even the ones you download from
them, a problem.
And collections are much easier on the Kobo than by jailbreaking your Kindle, so much as I hate steering anyone away from Amazon (

) I think the Kobo would be better -- unless you really like the Kindle and don't mind either jailbreaking or doing all your collections by hand.