Mechanical vs Pneumatic Stall Warning Systems
Electromechanical: 

Pro - can be tested easily during preflight by turning the mains on and manually actuating the tab

Pro - Potentially lighter (although not my much)

Pro - easy to install multiple switches in parallel if you want to trigger based on a stall of differing characteristics as is done on the Piper Seneca

Pro - Easy to wire into other systems. Maybe you want to add a stick shaker or some kind of feed back to the autopilot, much easier if its already generating an electrical signal. 

Pneumatic: 

Con - can only be tested by creating a low pressure on the horn, not as easy as the tab option

Many of the above pros become Cons for solid state units, like being harder to connect to other systems with out a transducer, harder to add multiple units without multiple horns, and the weight.