Read this review: Pumpenstein CCM - Series 6 Review
This emphasizes the differences between the S5 and the S6.
Also - Read this review:
This explains exactly what the S5 is about
: Pumpenstein Series 5 Basic - Ongoing Review
The SS-25 was a Mini'd Cocker body with a Spyder valve inside and a different front block to accomodate it.
It also had a body tapped to accept a Spyder feedneck. This was designed to make the SS25 modular so that it could accept a Stock Class feed. The SS25 was an attempt at a hybrid - a higher pressure marker, slightly more efficient, that could be run stock class.
The S6 is a sea change for CCM in their commitment to open class play and not stock class.
If I were you - I would buy a S5B or a S6 - if you have the money - buy the S6. It is the premier open class marker that is purpose built in my opinion.