Main differences I can remember are the Mk11 has a safety catch and a pressure guage. They also gave it a higher shot capacity by lengthening the air resevoir, Mk11 carbine is roughly the same length of a standard Mk1.
Cocking action is the smoothest you will find on any rifle,but is slow to operate (feels like it works by hydraulics). There are reports of it misfed pellets but as I only used the recommended accupels I never had any problems. New prices are quite high but second hand seems more reasonable because they don't seem to hold there price very well.
I would definatly be interested in buying a short .20 cal, but don't tell the missus
Common sense is becoming so rare these days, it should be considered a superpower