Disclaimer: I am the lead programmer for GrailsUI.
I'll give you 3 ways that GrailsUI is superior to RichUI.
1. Managed resources
2. Component accessibility
3. YUI config pass-through
YUI components can be created with many configuration options, so as a GrailsUI user, you can specify these options to the GrailsUI tag and they will be passed through into the underlying YUI component that is created. This allows you great configuration of your components without GrailsUI interfering.
RichUI also provides more components outside the YUI realm that are useful. I have used both of these UI plugins simultaneously in my projects in the past successfully.