To give me two cents:
Frankfurt - grimy, big red light district which you can Sexplore when not at the FKK if you like it really dirty and sleazy, the city is compact, it has multiple FKK clubs you can try. Also closer to the UK, hotels near the station are cheap.
Berlin - Huge city but a lot nicer than Frankfurt, much more 'cultural' stuff to do, great vibes, hotels can be cheap or expensive - however, only one FKK club to try and no RLD to sleaze about it.
I've done both Sharks (Frankfurt) and Artemis (Berlin) in the past year - I prefer Artemis, larger range of girls, in my experience just a better vibe, possible to get away from the cigarette smoke BUT be warned if you go in the winter when the garden is shut it can feel very compact. The food is miles better in Sharks, and Sharks tends to have less tourists.
I agree on both the latter half of the week is better - however, last time I was in Frankfurt a few of my regular Fuck and Suck buddies informed me that Sunday is currently the best day to go - which is odd because pre COVID Sundays were dead. Also, in both clubs, again different to COVID - the best girls and action seems to be later in the day so if you do go early to enjoy the facilities, keep some powder dry for the girls that will wash up around 7-8pm. Some of my all time fave punts have been with girls that rocked up to Artemis around 8pm!
I am also a greedy fuck, so I would say do both - one now and one in six months time
