Having a pump that is too big does not help keep the water clean. In fact, it does just the opposite. It pushes the debris through the filter with so much force that it doesn't actually filter anything.

A sand filter is easier because when you have to clean it, you simply turn a handle. With a cartridge filter, you actually have to remove the cartridge, maybe soak it to get it clean, hose it down. A messy job. You do lose some water with each backwash on a sand filter, but it isn't a lot. If you buy a large sand filter, you shouldn't have to backwash it very often. It is mostly just a preference thing. Some people prefer cartridges, but not many.

You don't even need a 1hp pump. A 3/4hp pump is plenty big enough and maybe a 200lb sand filter. Or even bigger. You can't get too big with a sand filter. (For comparison for you --- I have a 27ft AG pool which is approximately 18,000 gallons. I have a 1/2hp full-rated inground pump paired with a 300lb. sand filter. Even if you go with the 15ft pool, your volume will only be about 5500 gallons. You certainly don't need a big pump.)

There are other members on the forum who have more knowledge of pumps than I do. Let me see if I can get one of them to chime in here and give you more specific advice.