I could never get used to the pens. In my opinion, spotting dyes are the way to go! Spotone is unfortunately no longer with us, Marshall's works just as well. I have made my own from China Ink sticks and wouldn't hesitate to use good fountain pen ink as well.
I can offer this read on the subject:
and some interesting facts about fountain pen inks:
I suggest the following action list, sorted by my preference:
1. Get used Spotone #3 and #2 if you sulfide tone your images.
2. Get the Marshall's equivalent (still available).
3. make your own using an ink stick.
4. Use black fountain pen ink.
But whatever you do, don't use products with egg-white (such as Schmincke). They are also expensive and do not soak into the emulsion but end up sitting on top of the print where they are shiny and easily detectable.