If you decide fot the shower, speed up to the RCC, because there are 3 shower rooms and once they're taken, your name will be put on a waiting list.

Since your flight (UA 978, I guess) comes in 1 hour after UA 900 from SFO, there might still be a "backlog" from that flight. This will also determine if you have enough time to freshen up before you connection.

The F section at the FRA RCC is a separate room to your right as you enter. It is spacious, quiet and usually not very crowded.
A nice place to relax.

Good luck.