AA SLF
The difference between the Secret Service man and a Sky Martial is that the Sky Martials are specifically trained to operate within the confines of an aircraft cabin. That means that they are aware of the potential for an explosive decompression if they discharge their weapons. As I understand it they also operate as a team rather than a sole agent.
As an (unarmed) UK Police Officer I know which one I would prefer to be looking out for my interests on a flight