Pretty much every ePub device - with the exception of the iBooks App for the iDevices - uses the same firmware to display ePub files: Adobe Reader Mobile. So from the viewpoint of displaying ePub files, it really doesn't matter what device you get. However, there are obviously many other considerations when purchasing a reading device, and this forum is the best place to get that advice. One very important piece of information which you haven't told us, for example, is where you live. That very much influences what devices are available to you.
I would note, by the way, that converting Mobi to RTF is a truly awful way to edit Mobi files. You'd be a lot better off converting from Mobi to ePub, and then using an ePub editor such as Sigil to edit your books.
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