The Helix is a pretty different animal: detachable transformer style keyboard vs. the slider. Honestly, they're both really nice machines, and I think the form factor is a big decision point (and a matter of personal preference). The pure tablet option is attractive to graphics artists who typically hold the tablet a lot when drawing. The every-ready keyboard on the Vaio Duo 11 appeals to those who type a lot (though the keyboard is cramped).
One drawback to detachable tablets is that they generally throttle down the CPU more since they have less room for cooling. Lenovo handles that in an interesting way where the CPU runs at full power when it's connected to the dock (since it adds 2 cooling fans).