Why BookMacster? Besides doing everything that Bookdog does (although some in different ways), BookMacster supports Google's Chrome web browser, and also new Delicious accounts linked to Yahoo! accounts. We intend to not add this support to Bookdog.
Why Bookdog? The only reason to use Bookdog instead of BookMacster at this time is if you really like the dob motif, or if your Mac runs Mac OS 10.4. BookMacster requires Mac OS X 10.5 or later.
Pricing for Bookdog and BookMacster is the same, and when the introductory pricing for BookMacster is increased, pricing for Bookdog will be increased to the same level.