During our recent trip to central New York to celebrate my dad's 90th birthday -- which was quite a celebration! -- we picked up a couple of packages, one of which was a new extra-fine nib for my Lamy fountain pen. When we got back I changed the nib and did two drawings of the same subject, just from slightly different angles, with my favorite fountain pen (top) and with the Lamy (bottom). While I still prefer the flexibility of the Sheaffer nib, I'm quite happy with the Lamy now.
Then I added some watercolor; the fountain pen ink (Skrip cartridge) isn't permanent so you get blurry lines, while the lines drawn with the Lamy and Noodler's Lexington Gray ink retain their crispness. I don't think it's a case of either/or - they're both nice, just different -- do you have a preference?
On another artist's website I read that you can mix inks in an empty fountain pen cartridge to get the color you want -- a great idea that had certainly never occured to me. I'm not going to be putting permanent inks in the valuable pen, though.
Spaking of permanent inks: this bottle of Yasutomo sumi ink, in its characteristic "jade green" container, is one I've had for ages. The most meta thing to do would have been to draw another sketch using a dip pen and the sumi ink...but that's a bit too obsessive, even for me!