You can choose the one or the other both are suitable , non of them will work as a good (full) flex pen without tweaking, and this is a fact, currently you can't get a good flex experience out of the box from a modern pen.

Modifying the nib is quite easy, tweaking the feed is the difficult part.
I suggest to read the EMF thread on FPN I linked above.

Or go the dip nib path first....