Onager or Kastelan?

I got the newer combat Patrol recently and also have an extra unit of vanguard. I'm able to justify buying a new unit and I like how the Kastelan Robots look as well as the Onager Dunecrawler. I am currently not sure which one I want to get and I don't have a great grasp on what currently is good, I was wondering which people prefer or which is better?