Until recently I would have said there is no doubt that MOBI is winning the availability war. This is certainly true in the US, but in the UK there seem to be many new titles in ePub only.
One interesting comparison is
WHSmith eBooks in the UK and
Books on Board in the US, because both use OverDrive to provide their ebooks. WHSmith is an OverDrive
MIDAS (ready-to-deploy eRetail) store and BoB uses OverDrive as one of several sources for its ebooks. Note that BoB gets MOBI ebooks both from OverDrive and from MobiPocket, so it should stock the title if there is a MOBI version. I assume WHSmith gets its MOBI ebooks exclusively from OverDrive.
The current WHSmith top 10 fiction tiles are almost all Adobe ePub only, and most seem to be UK only as well (i.e. not available on BoB). For titles that are initially released in the UK and then released in the US, I assume there may later on be a US ebook with a MOBI version. However, there are probably UK authors that are never released in the US.