This is, as in all such things, a matter of personal individual choice.

SQ25/26 always have the aircraft parked way out at the "C" Gates, which at FRA represents an "Extra Security Zone" for all non-LH flights To/From the US. Because of this, I always found the RCC more convenient than the LH Senator lounge. The F side of the RCC was very comfortable and usually empty so getting a shower was never a problem in transit.

Conversely, at peak hours, The Senator lounge(s) are completely full, often with nowhere to sit. To take a shower, there are only a few cubicles so it is necessary to put your name in a book and join a (Frequently long) queue.

Anyway, the real solution when flying out of FRA in F is to fly LH/F and use the separate LH/F Terminal.