I have an Aluratek Libre Pro and it supports both. On it I'd take mobi over epub because the progress also shows up in percentage instead of just page numbers with mobi and also because mobi ebooks don't have the page numbers show up over the text on the right margin of the screen. Most of the time I convert everything to fb2 because it works great on both my and my Hip Street ereader or to rtf when I want to use my PDA with ubook.
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