I prefer the Senator lounges over the RCC at FRA.
• The Senator lounge had far superior snacks (the sandwiches rotate every hour or so; fruit selection is fairly ample; wines and other drink selections seem better to me)
• A massage chair and other recliners in a semi-secluded room. If you get there early (first come first served!) and have a long layover, you can get some fairly decent rest. RCC has nothing like it that I know of (never seen the first class portion).
• Senator lounges have a better choice of electricity source in their business kiosks. Most of them are wired with 110 volts as well as 220, which can be a battery-recharge lifesaver for Americans. The RCC inexplicably has nothing for 110 volt users except a sign saying that the on-board power has been disconnected on the 777s and that you should recharge your batteries while on the ground. There are only two kiosks in the RCC if I'm not mistaken, compared to anywhere from 4 to 12 in the Senator lounges.
• The Senator lounge on the B concourse has a selection of different TV channels running concurrently, with headsets so that the noise isn't blaring across the room. The RCC has only one TV in a corner, viewable to only three or four people at a time.
• The shower facilities at the Senator are fairly neat, clean and orderly. I have yet to try the RCC showers, so I can't say how they compare.
• The Senator lounges are much closer to most departure gates, meaning that you actually get to relax about ten minutes more. And beware staying in the RCC up until the last minute if you don't know your way around FRA. Getting from the RCC to any other departure gate can make you feel like a mouse in a maze if it's your first time around.

On the other hand, checking in for a UA flight at the RCC can be much superior. Questions about domestic connections in the USA are obviously better asked of RCC people. Be careful of the timing. The RCC is staffed by true UA employees until sometime in the late afternoon. After that some gracious staff take over to make things comfortable, but they are not UA agents and cannot answer any questions.