Fountain & rollerball ink is water based. Ballpoint is oil based. If you write a check with a ballpoint pen, there is a risk the check could be altered with solvents (i.e. the oil based ink could be washed away with solvents) where this is more difficult with water based inks like those found in fountain pens and rollerballs. Personally, I don't see the point of a fountain pen. Even with a cartridge, a fountain pen is less convenient than a rollerball. And a ballpoint writes less smoothly (otoh, ball points won't dry out like a rollerball and write over a wider variety of surfaces, my advice, keep a Fisher Spacepen to cover difficult writing conditions, and a good roller ball for every day).