Ranga pens these days can be bought as both eyedropper and cartridge/converter; if you go for the second option (which often doubles the cost of the pen) you have the additional option of not using the cartridge or converter, thus getting a pen you can fill three ways instead of just one. Note that this is regardless of the question of whether the nib & feed are friction-fit or screw-in. Ranga's, AFAIK, are all friction-fit. The difference is in the feed, which for the cartridge/converter option has the nipple that connects the ink reservoir, while the simpler system just sticks the end of the feed into the ink.
Having used both types of feed in eyedropper-filled pens, I have not observed any difference in flow between the two. If obstructed by for example solid matter (fibers, or dye, etc) both systems will exhibit skipping, hard starting, etc. And I often find the simpler nipple-less pens easier to clean by blowing water through them, although that may be pure prejudice. One thing is certainly superior in the simpler system: when you prime the pen the effect lasts longer.