There is the SotS Ultimate Collection on Impulse for $29.99 (which is just four cents more than Sins) which has SotS plus both Born of Blood and A Murder of Crows. Great deal if you ask me. Plus there is a micro-expansion for AMoC, Argos Naval Yard, releasing on June 17 and priced at $8.99 which adds 10 new weapons and over 75 ship sections spread out over the 6 races.
For racial diversity and backstory, SotS really takes the cake. On the Kerberos forums, there is an entire sub-forum dedicated to the various races in SotS. You could spend a couple hours poring over the backstory of the various races, and more is being added all the time as the fans ask the developers questions, and the developers answer them. The amount of developer-fan interaction is really something. The developers take the time out of their busy lives to post on their forums. And I don't mean like it is here where they rarely post, the lead developer for SotS has over 13,559 posts on the forums since August 5th, 2005. The SotS developers definitely listen to their fanbase.