Bag filters are cheap and are generally used by people that don't know any better. The bags are typical rated as "nominal" micron size, meaning that about 65% of the "nominal" micron sized particles are retained. Of course, you can also specify an absolute filter, but then you will suffer with low capacity.
Cartridge filters are more expensive, but will allow higher capacity, more predictable performance, and longer filter runs because of in-depth filtration.
Both bag filters and cartridge filters are only suitable for low solids polishing applications. Otherwise, the performance will suffer.