I am a bit late to this thread but I'll chime in with my 2 cents. Disclaimer: I am not an expert at flex and I'm still looking for a good vintage flex pen.
That said I recently bought a Binder XF/XXF duo nib full flex and I got an Arco Milord with a Portico F nib (14kt) modified for some flex.
Keep in mind it is comparing apple to orange since the nib/feed unit of these 2 is very different in design and in material (plastic for the Pel and ebonite for the Omas feed).
Here's my finding:
The full-flex Binder nib requires quite a bit more strenght to flex, not having been told I would have thought it was semi-flex. On the other hand the size of the M250 nib is smaller than the Portico so I'm sure it limits what can be done, i.e. the amount of material that can be safely removed and the natural springiness coming from the length of the tines.
The Binder nib has
exceptional flow control, I can flex a lot and let go and get and XF line right there. The way the nib/feed react to flex is exemplary but it requires enough strenght to be tiring for the light pressure writer that I am. On the other hand the Mottishawed nib flow is problematic after flexing I get a LOAD of ink and so it removes much line variations I would get I had requested a relatively wet nib so the fault here might be mine for getting
what I asked for the nib will be going back and I'll be able to give an update afterwards.
Here are some pictures showing a bit of these nibs (please excuse my in "retraining" handwriting). You'll see that the more the page progress the worse my handwriting gets due to getting tired:
http://picasaweb.google.com/PenWritings/FlexExemplesOne more thing to consider, the body of the Milord beeing much larger it may provide more of a lever effects hence why the Arco felt easier to flex than my Binder XF flex nib in my M620.
A short description of the Binder XF would be "striking line variation"
for the Mottishawed Arco it would be "smooth and wet flex"
The Mottishawed nib is much more pleasurable to flex, more progressive and less tiring on the end so I'm especially curious to see how it will turn out after beeing adjusted.
I had never used XF or finer nib. The Binder XF/XXF has a lot of feedback, much more than I expected, but I guess it makes sense because there is not much tipping material left with a nib of this size! When debating which size to order I was debating between an XXF/XXXF duo full flex or an XF/XXF duo full flex and went with the later. From what I can tell the difference between upside down (XXF) and (XF) is minimal, I can see it, but there is not much difference in term of smoothness so I would say go with either they will be equivalent except the XXF will give you a slightly lighter line, slightly. I am sure that XF/XXF/XXXF etc experts can appreciate better than I their differences.