I am a bit at a loss to understand the implication that ePub is somehow a Sony format. In fact it is actually the effective world standard format, and almost all eReaders of whatever make (with the notable exception of the Kindle) support it.
If you visit my blog on eReaders (www.ebookanoid.com)you will find loads of eReader reviews, and they all work happily with ePub.
Further, the equally strange idea that a lot of Sony eReader owners seem to have, that they can only get hold of eBooks on the Sony eBook site is wrong. I have owned two Sony eReaders for two years now,and have never been to the Sony book store. You can purchase eBooks online in ePub format at dozens of eBook sellers - Barnes and Noble, Books on Board, Waterstones,Borders and so the list goes on..... Choice enough, regardless of what sort of book you like to read.
So, look for an eReader whose specs seem to meet your particular requirements, make sure it will take ePub, and buy it, and have fun shopping allover the web for your eBooks.