In Wyoming and Montana it has been harder and harder to find the square head screws. The torx screw seems to have become quite a bit more popular. The torx screw head strips out easier than the square head but not as bad as the phillip head.
When putting screws in with a power driven device, you don't have to put as much pressure on it if it is square drive as opposed to phillips in order to keep it from stripping out the head. This reduces fatigue if you do this all day long for a living. That is why they are used extensively in the RV and manufactured housing industry.
The strength is the same.
I prefer the square drive. At one time I could get them at Home Depot. Now I have to go to Grainger or a trailer parts place.