For the most recent ebooks (from the last year or so) there is usually little difference between ebooks formats, but the original "master" format is usually ePub (as used by the Adobe ePub version). The Kindle version is MOBI which is usually close to the original ePub. The B&N version might be ePub, but is often eReader - which usually is formatted more simply than ePub. I have the impression that the B&N ePub (when available) might in some cases be back converted from eReader which does not improve the formatting over the eReader version. For an example, see
A Comparison of format limitations based on The Hobbit.
On the DR800SG, eReader ebooks probably look close to Desktop B&N Reader. However, ePub ebooks (perhaps those bought from B&N but certainly ePubs bought from all other sources that sell the standard Adobe ePubs) will look closer to what Sony eBook Library Reader displays (see
eBL Download). This is because the eBL is using mobile Adobe Digital Editions for ePub and PDF, just like the DR800SG. The screen size (in pixels) is different though, with the eBL using 600x800 like the 6" EInk Readers.