Schedule 40 dominates the market utterly.
It is used above ground and underground. Wherever you might think to use plastic -- Schedule 40 PVC 'owns' the market.
Schedule 80 is used but rarely. It has a tighter interior dimension and costs more. Hence it is only used when the raceway is exposed to damage.
I work commercial. If any pipe is at risk for 'damage' it is protected by bollards, etc. Schedule 40 rules.
Schedule 80 is practically never used underground. What would be the point?
