Some of the changes from the SX-12 to the SX-15:
Handle lays down flat, deck is lower. These two changes make it much easier to get under things from toe kick to tables.
A single 2" vacuum port. This increases CFM and the larger vac opening is not likely to get plugged with any debris.
Different vac port placement and design of the inside of the head creates a
vortex effect with laminar air flow across the floor to dry faster.
The vacuum relief can be easily changed even with the toe of your shoe.
No tools, no hot water required to change from brush bottom to a solid plastic boot. Can be done by most folks in 1 minute.
25% larger cleaning path makes the cleaning faster.
Heavier duty design feels more solid