I suspect people are probably doing west only to cover air conditioning operation. I dont have any data myself, as my W horizon is well over 20 degrees up, due to living in a valley, so I couldn't get any valid comparitive data.
For an unobstructed horizon, west facing will generally produce slightly less than east facing, due to the higher operating temperatures when the sun is near face-on. Around here, and many inland areas, there tends to be more cloud in the afternoons- cloud formation is loosely correlated with maximum heating, so that could reduce output a bit too. Patchy cloud any time of the day can increase output due to cloud edge effect too, so it is quite complicated to work out for a particular location, without actually collecting data.
Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries