You van use VWD 2008 to develop commercial use apps. see the item no.7 in link below
http://www.microsoft.com/express/support/faq/
As for difference..some comparison here for 2005 .Almost the same difference applies for 2008 , but I could not find the official version
http://msdn2.microsoft.com/en-us/vstudio/aa700921.aspx
Jeev
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you get the answer to your question, please mark it as the answer.