I thought Former Marine is one no longer on active duty and/or no longer in the Reserves and Ex-Marine is someone who dishonored himself by dishonoring the Marines/the country, i.e. Murtha.
“no longer on active duty and/or no longer in the Reserves” is a Veteran Marine. The likes of Murtha is where the argument between former and ex come in. It may be just semantics, truth be told.