I own both a Kindle Basic 10 and a Kobo Clara HD.

Both devices can sideload books just fine out of the box, and you will be able to read them without having to do any hacks or jailbreaks. The easiest way to sideload and keep track of your books is using Calibre on a computer.

But I will say that the sideloading experience of the two devices are night and day.

Kindles are very clearly built to funnel you into the Amazon book store. Buying books from Amazon is smooth and easy.

For sideloading on Kindles you must convert to mobi, azw, azw3 or kfx. All of these have different feature support. So if you want Book covers, the updated layout engine and typesetting, then you must use kfx. But Calibre can’t natively convert to kfx. So you will need to install amazons ebook previewer and a plugin in Calibre to make Calibre convert to kfx via the amazon ebook preview application. Each conversion takes roughly 2 minutes, and randomly fails for no apparent reason.

If you decide to use Kindles’ email option for sideloading, then your books will be converted to mobi, so you lose out on a lot of features. And the kindle sees the books as documents, not books.

If you sideload with Calibre and try to upload books with book covers, then it will work fine, and for a couple of seconds after uploading the book it will work fine. Then the Kindle will realize that should definitely look up the book cover om Amazon, and if it finds the book if will overwrite your book cover, if not it will replace it with a blank page. You can then reconnect your Kindle to Calibre and Calibre will fix your book covers properly. But if your Kindle is able to look up the book on Amazon it will continue to overwrite your book cover.

Finally the organization of sideloaded books sucks on Kindle. If you sideload via email, then you can organized the books through Amazon’s website. If you sideload with Calibre you can’t, and your only option is to manually organized your books into folders on the device one by one. This is extremely slow and tedious.

Sideloading books on a Kobo can’t be done via e-mail, but Kobo supports epub out of the box, which most ebook are. If you want the books to load and navigate faster, you can convert to kepub, this requires a plugin for Calibre, but no additional software. Each book conversion takes 2-3 seconds, and the book arrives on your Kobo with a functioning book cover, full functionality and zero fuss. Additionally Kobos automatically organize books into folders based on both author and series based on your metadata in Calibre, making it a breeze to organize your entire library on your computer and just transfer things, already organized, to your kobo. Kobos also has an additional section called “Collections” which you can map to any field in Calibre you like. I have mapped mine to a Genre field, but you could organize stuff by anything you want.

So if you are planning to primarily sideload books, I would strongly encourage you to look at a Kobo instead of a Kindle.