I actually have no idea how people can use touchpads. You need a huge one to be able to go corner-to-corner in one swipe and still have reasonable accuracy.
I mean, given the option of a physical mouse, I will always prefer that over both nub and track pad -- and the physical mouse requires the most movement away from the keyboard (of course, to avoid all three mouse input methods I use keyboard shortcuts when possible).
Anyway, my reasons for preferring the nub are:
* Less movement required on the actual input device. Just a slight tilt of the finger can move the mouse cursor from anywhere to anywhere else.
* Higher accuracy. This one may be subjective, only personal applicable, and/or biased, but I think the nub just makes it so much easier to navigate compared to the touch pad.
I use the nub on my thinkpad, but only because Lenovo's touchpads are truly woeful.