DjVu is a GRAPHIC format. it has the text layer, but it is not displayed and need to be able to copy the text. DjVu it is possible to overtake in pdf (for the Knicks there is a script djvu2pdf), the page images are converted to a graphics format used in the pdf and saved in pdf with the pages-images.
The Epub container containing first text in the form of html pages. Due to this it supports complex layout, graphics, but you can change fonts and font size, and pagination is not fixing. On the epub standard can contain and pdf, but in fact it makes sense when the book is assembled from several pieces(say, several chapters in PDF Oh and the other in html).
Since all the e-books support the ePub and pdf support (since the to display ePub now in all e-reader using Adobe Reader Mobile 9 SDK) that make sense to convert pdf to ePub no.