It all boils down to one simple question: what is more important of large ink capacity or ease of cleaning?
If you are the kind of person that has a lot of different inks, and like to change them on a frequent basis, then having room for a lot of ink might not be the most important. Because cleaning a cartridge converter pen is quick, you can just flush it out with a bulb syringe and flush the converter and you’re set for a new ink. While doing the same with a piston filler takes longer time because you need to fill and empty the pen until it is clean.
On the other side, if you are like me, and often use the same ink for months, if not until the bottle is empty, then you probably prefer having room for a lot of ink. A Piston filler is made to have room for as much ink as possible. While a converter is often designed to fit in a large number of different pens. And the result is often far smaller ink capacity.
I have pens with both. But I always prefer room for more ink because all of my pens (except for my Noodlers Ahab) is filled with the same ink. And I use a pen until it runs out, then I move over to the next pen, refill the empty one and rotate through all of my pens.
And I try to clean them every second to third refill or so. For me it doens’t really matter how easy it is to clean them for a new ink because it don’t do it that often.