System 3 cartridge systems are -- in most locations -- just about the worst possible option. I'm amazed you've stuck with one for 20 years (I didn't think they'd been around that long).

Your location and local codes determine the best options, in many respects.

Sand filters are BY FAR the easiest and most durable, BUT you have to be able to dispose of backwash without running afoul of regulations and restrictions. DE filters -- even the StaRite DE filter that uses the same case -- produce noticeably clearer water, but in shallow pools the difference is not great, and they do best if the pump runs 24/7 (low speed is OK).