Should I use a cartridge filter or a sand filter for my pool?

Go with a cartridge filter. It catches particles down to 10–15 microns for clearer water, uses less water (no backwashing), and costs about the same over time. The only trade-off is physically cleaning the cartridge every few weeks, but that’s 20 minutes with a hose. Unless your pool is near a forest with heavy debris, cartridge is the better investment.