I've had both, but not at the same time.
Also, I used the pico w/ a PC only, and Duet w/ Mac only obviously.
Honestly I dont think you'd notice a difference, except that at one point I had to keep re-syncing the Duet, it became distorted. "Refreshing Connections" didnt solve it, but toggling from 16 bit to 24 bit, then back to 16 bit did the trick (you can hear the hardware make a loud click sound each time). The annoying part is you had to close itunes first.
I've not had this problem since bringing it all home from the office for some reason.
But I never had any such problems with the pico, it was always rock solid.