An S6.5 has the Spyder feedneck/base that you can remove and have a clean CCM SC feedblock attached.
A half blocked S6 is a travesty and whoever hacks up a such a beautiful marker should be hanged up by his toes.
The S6.5 also has a longer hammer to make up for the bolt pin slot. The stock S6 hammer is shorter otherwise they are identical mechanically.
All a matter of preference and feel. Some people to play tight so an S6.5 helps. Some people say the S6 is smoother but again all personal opinion.