In general terms better to use larger mowers. However the best results is when the equipment used is best suited to the job. Also using a 21" when the job is priced right will make a good profit. Hill? What hill? That's why you only buy 21" self propelled. Retaining wall? What retaining wall? All the people that start with a 21" and load the mower in the trunk of their sedan should be able to get a 21" over a retaining wall.