Both have their bonuses.
Cartridges are clean, simple and require zero maintainence.
Bottled ink, however, adds further to the enjoyable experience of writing with a fountain pen. It makes it feel more of an occasion.
But, by god, it's messy, dirty stuff that you will get absolutely everywhere. Cartridges are far better but, then, if we cared purely for functionality, we'd use a Bic rather than a fountain pen.
Also, I wrote this message on a loom.... pip pip!