Of the two filling mechanisms you are the most likely to find in any random fountain pen on the market I prefer piston fillers.

A big part of why is that I have no interest in any ink I have ever seen available as a cartridge. Partly because of the expense, but as much because of how few inks are available for the various cartridges.

That leaves me with either a converter or eye dropper filling the pen. The latter is a little bit messy and the former is either the same as a piston just worse in most ways. Or as messy as eye dropping it if you try to make it better by filling the converter with a needle.

A converter is like a piston filler. Or most of them are a piston filler on the inside. But they have less room for ink since they are inside the pen. And they are usually worse at being able to fill the entire room with ink plus you have to unscrew the pen to get to it.

The bad thing about a piston filler is that it is usually a bigger hassle to clean it than with a converter.