I have both a Summit and Zen, definitely compliments to each other for me, Zen is about 4-5 boards weaker and smoother with the same layout.
Zen vs Duo is interesting.
To me, they fit in the same spot but do things slightly different.
Microtrax cover on Duo is a little stronger than the Zen's cover. (Zen also has Nano, most people aren't aware but Schlem has mentioned it multiple times. Gold Label is no Nano).
The Zen core is slightly stronger than the Duo core.
Since I've drilled my Summit, I've actually rarely thrown my Zen because of how versatile the Summit is. Lately when I've balled down from the Summit, I've gone straight to my X-2 if I need length and shape or my Tour DynamX if I need control.