Frankfurt is easier to get to. The trains to the city run about every ten or fifteen minutes and then it takes about twelve minutes to get to the main station.

You can get to London in fifteen minutes on the Heathrow Express but it's £34 or so per person. Otherwise you can get the much cheaper Piccadilly underground line which brings you straight into central London but takes just under an hour.

However, London is much more interesting than Frankfurt.

Which flight would you be looking at getting from FRA? The LH JNB flight leaves quite late in the evening.