As for any filters you really can't go too large. The slower the water flows the better the filtering. Too small of a filter for the pump and dirt can get rammed through. I'd go with the 24 if the price difference isn't that great. Not sure what the sand weight is for the 24 though. As for reliability I have a 300# that's going on 14 years and the only thing I've had to replace is the spider gasket in the multiport valve.
Al