ePub ist more universal right now, but Mobi/PRC still has its merits.
And as long as you don't restrict it with any means of DRM (which you might choose yourself when creating the files via Mobipocket Creator), it is IMHO about as flexible as ePub, since the software is freely available.
Simply create your eBooks in TWO formats, ePub and Mobi, and you will please the majority of readers out there.