The biggest difference is in the amount of adjustment between the cartridge kit and the piston kit.
The piston kit- only replaces the fork piston and valving along with it, the amount of adjustment is determined by all the other hardware in your forks.
The cartridge kit- replaces every thing inside the forks, (pistons, valves, compression tube, cap, needle, springs and all.)
You will gain over all adjustment with the cartridge kit VS. the piston.
I.E. 20 clicks of rebound VS. the showa standard 10, the over all adjustment is around the same I would assume but the increments in which it is adj is smaller and finer on the cartridge kit. allowing you to more fine tune depending on track and other factors. same goes for spring preload and high and low compression.
You will also gain travel with the cartridge kit, because it eliminates the top out springs. I.E. giving you more adjustment to better set spring preload and over all front end feel.
Also with the cartridge kit you have much higher manufacturing tolerances for the rest of the fork components which are not replaced with the piston kit, which means less friction, deflection, oil bypass and others which leads to a more constant set up.
All that in mind, I would say if you are a suspension set up novice or slow like me, then you would be better served going with the piston kit. Although the cartridge kit will bring you a higher resale value then the piston kit ever would
DISCLAMER: I AM SLOW AND DUMB
( I ride a set of 25mm cartridge= I am partial to them, but I have installed a few race tech piston kits, set up a few ak-20's, and rebuilt more forks then I can shake a stick at.)