I was not in Berlin so can’t really comment on there and Frankfurt was quite similar to London in many ways.
I found that the Germans were arguably slightly more friendly but coming from London originally I am probably not the best judge.
I would expect London to be slightly more chaotic and fast paced (in a good way).
Languages definitely help and the value will range depending on the type of work you are doing and the field you are in. We employ native speakers (or at least very strong) in 2 or 3 languages and they are often very hard to come by.
Rents are unlikely to shift a massive amount and as people have said if you are prepared to commute then its not that bad. It is important to consider how close you want/need to be to the center and the difference between the types of places and areas you see when visiting versus where you may end up living (obviously for all I know you could be looking at 100K /year jobs and could live in Chelsea!).
That said there are numerous areas slightly further out in most directions which are really nice to live in.
Probably the best advice would be to revisit both with a clearer idea of where you might want tol ive and what you want to do. go for a little more time, maybe use Air BnB or similar and actually live life in a flat etc. like you would and treat it less like a holiday.
Do you know what you want to do if you came here? What other languages do you have?