I have the Mk25 with the S555 second stage. I am very happy with it. However, for cold water diving (45 degrees and below), all reviews and divers I have spoken to recommend the Mk 17 over the Mk 25. Price wise, the Mk 17 falls somewhere between the Mk 11 and the Mk 25. The S555 is the same second stage as the S600 without the airflow adjustment. In the future, I will probably purchase a S600 as my primary and use the S555 as my backup.