I've owned a couple of Chromebooks, and while they are for the most part similar, there are a few things that make a big difference in performance and improve the user experience.
1. A display with good viewing angles. Some people prefer larger screens and some people prefer a smaller laptop instead. However, I've never met anyone that likes poor viewing angles. This is a common issue in inexpensive laptops, not just Chromebooks. The only way to be sure that it is acceptable is to see it in person, so make sure that you actually look at the screen that will be used for hundreds of hours before you make your decision.
2. Intel inside. Chromebooks with Intel CPUs simply perform better in my experience. ARM Chromebooks have gotten better over the past year or so, but I'd still go Intel.
3. 4GB RAM. I've used Chromebooks with 2GB of RAM, and they would start chugging along as the number of open tabs increased. It makes a huge difference in the user experience, perhaps more than the screen quality does. Seriously, spend a little extra for 4GB RAM and the user experience will be far superior.