Stability ???
As a PAX I can certainly "feel" a difference between the A319 the A320 and the A321 which I have never really been able to sense in such a clear way in the DC9/MD80 or in the DC10 & 747 families.
The A 319 seems to twist left & right around the center point more than the A 320, whereas the A 321 seems to pitch up and down around the center point more. The A 321 also shudders as it takes off just after rotation, which I have so far never noticed on the smaller planes. :o