What each licence lets you do
Security Guard covers static guarding, mobile patrol, retail loss prevention and lock-and-leave — anywhere alcohol is not being served under a Premises Licence.
Door Supervisor is a superset. Anywhere a bouncer would stand, that's Door Supervisor territory: pubs, clubs, festivals, hotels serving late-night alcohol, some ticketed sporting events.
Cost and time compared
Security Guard: 4-day course, physical intervention not required. Door Supervisor: 6-day course, physical intervention module included.
Licence fee to the SIA is identical for both. The extra £60–£90 in course fees pays back inside the first shift for anyone who ever considers nightlife work.
When Security Guard is genuinely the right choice
If you have a documented medical reason that prevents physical intervention training, Security Guard is the appropriate route. Otherwise, the industry default advice is Door Supervisor.