There's zero inconvenience barrier between my fountain pen with ink cartridges and my rollerball pen with ink refills. Writing and replacing the ink cartridges

There's zero inconvenience barrier between my fountain pen with ink cartridges and my rollerball pen with ink refills. Writing and replacing the ink cartridges are functionally equivalent between the two. But my fountain pen has a 1mm stub nib that gives the lines some distinct width, so I use it when I want that characteristic.