Which Chromebook is better?

I would say the Chromebook Duet is better.

But with a Chromebook you have to keep the following in mind:

1. You're tied to Google. Chromebooks aren't privacy-friendly.
2. If the Chromebook model doesn't happen to support the Linux beta, you can only install from the PlayStore and are very limited in your program selection.
3. Both of the laptops you selected have processors that are typically found in smartphones, so they won't be able to handle very resource-intensive tasks.

I still recommend getting a regular laptop and installing Linux on it. If you absolutely have to go with a Lenovo, ThinkPads are also a good choice.

If it absolutely has to be one of the two Chromebooks, then the "Duet" is the one.