If you are a statistian, you would come up with more scientific comparison methods but nobody understands anyway.

Unless you have been the majority of RCC and MLL, it is not easy, and a lot times at different locations related to routes, the quality of these lounges varies. You can always find Krislounges from SingaporeAir are worse than RCC, but Singapore Air has many excellent lounges.

But overall, I have always felt MLL are better than RCC. I have used 17-18 MLL (except CDG's), and overall I have believed that are better on average (again here is a mathematician's buzzword, "on average") than United's RCC, which I have used something like over a dozen locations.

Most US carriers' lounges are cut to the bones for the services, but as many of the American hub airports have been worse than zoos with the crowd and crapy vendor services, these lounges still offer at least a quiet space to escape the noise and bumps.