Here's a quick image to compare the two pens first.
As you can see from the images, the 1.1mm nib writes a bit thicker than the 0.9mm nib. This is obvious of course, it's meant to do that. However I found that I liked the thick-thin definition of the 0.9mm nib a bit more than the 1.1mm, the contrast between vertical strokes and horizontal strokes appears to be better. I found this a bit unusual in that I expect a wider italic nib to have a better thick-thin ratio.
However there are other factors to consider other than just the line width and the thick-thin ratio. I think the ink used may have some effect, perhaps the Caran d'Ache ink I had in the Ventidue was feathering a little bit more than the Visconti ink in the Duetto.
In terms of "nib feel", these two nibs behave quite differently. The 1.1mm 14k nib was very smooth, like drawing a hot knife through butter. However, the 0.9mm nib just feels better, like a sharp knife carving into ice. This difference in feeling is quite obvious, and I definitely prefer the titanium nib.
Another factor to consider is that the titanium nib is a bit semi-flex. This may explain the better thick-thin ratio seen in the 0.9mm Ti, because the nib will flex open a bit during vertical strokes but won't be flexing for a horizontal stroke.
Edited by MYU, 01 December 2008 - 01:58.
corrected image links