Type substitution vs. element substitution (aka substitution groups)

Dear colleagues,

Is there a suggested best practice to choose between type substitution 
and element substitution for XML schemas that wish to support extension 
via inheritance? Please share your opinions and experience as well as 
any resources on the subject. Thank you.

-- 
Regards,
Farrukh Najmi

Web: http://www.wellfleetsoftware.com