Im a former Interspire-customer, but decided to go with CS-Cart when they focused more and more on BigCommerce.
Still, there is one thing that impress me with BigCommerce, and thats their great selection of third party apps. I really hope CS-Cart could make something similar with the new API hopefully coming up soon. Take a look at the BigCommerce Apps here:
http://www.bigcommerce.com/apps/
Wold you like CS-Cart to make a similar page?