As someone who runs Linux and ChromeOS on my personal laptops, was looking at the XPS, and just ordered a Pixel, I can answer that:
1. Chromebook is zero-hassle. It's great that Dell is putting native Ubuntu on their laptops, but fundamentally those are still machines that were designed for Windows first and awkwardly adapted to Linux. They'll always have a few niggling issues that a designed-for-ChromeOS machine won't have.
2. I prefer ChromeOS to Linux. The ultra-minimalist ChromeOS UI is just amazingly clean and elegant, in a way that none of the Linux desktops are (and I say that as a huge fan of GNOME 3).
3. The high-DPI version of the XPS 13 is more expensive than the Pixel (and might not be the one that gets released with Linux, they haven't announced it); high-DPI is a mandatory thing on an expensive laptop now for me.
4. 3:2 screen is the perfect aspect ratio, and the XPS isn't that.
5. USB-C charger is sweet.
But the XPS 13 looks like it'll be a nice laptop when the Linux version comes out. Still, I wish Dell would design Linux-first laptops rather than trying to awkwardly retrofit Linux onto designed-for-Windows machines.