cast iron works best but it is not absolutely necessary. I get great grill marks with stainless and whatever the standard grill is on the Primo. The most important thing is to make sure the grill surface is extremely hot before the meat hits it. Marinades with some sugar will make it easier.
Ruth's Chris is not all that to be honest. As far as expensive steak houses go, it is bottom tier in my opinion. I certainly would not turn down a free meal there, but if I am buying, RC is my last choice.
Primo Oval XL, BPS drum, Weber gasser