My understanding is that firmware does determine the type of formats that a device can read. I believe that I read somewhere that the Astak line is moving away from Mobi and towards ePub DRMed content. Robert can probably address the validity of this statement. I'm pretty sure that Mobi insists on being the exclusive format for a device if their DRM is to be used. This represents at least one good reason for a move towards ePub.
One of the other key points about the Hanlin V3/BeBook/EZ Reader family that I like is that the
OpenInkpot project supports them. OpenInkpot is an open source firmware project that has a heavy presence on these forums. They're continually adding features and support for new formats from what I can tell. Also, the stock firmware for these devices seems to be updated very regularly, so support for formats at least has a medium for improvement.