One thing I would add to Carl's list is that if you decide to go with a cartridge, I would oversize it so that you do not have to clean it that often. The unit I have has 4 cartridges and I only need to clean it once or twice a year which takes about an hour to do. In fact, I end up cleaning the filter before I see any pressure rise.

One thing that forced me to a cartridge was the requirement of our city to use separation tanks with DE or sand filters. I guess this is to both save water as well as prevent pool water going into San Francisco Bay. Anyway, I might have gone with a DE filter had this requirement not existed. But after dealing with a cartridge for a year, I am not sorry I have it.