by saying F from FRA, I presume you are considering a Lufthansa F?

If so DONT DO IT

I recently flew F on Lufthansa return JNB-FRA, it was terrible! From cabins that are throwback to the 80's they feature CRT tubes for crying out loud!, poor IFE, and so so meals, to overhead lockers that can barely swallow a pair of shoes!

The only reason to consider Lufthansa F is the First Class terminal in FRA (which you skip if you fly in from another city).

SAA C will be far better, comfortable (i weigh 130kg, 6ft tall) and easily get a good nights rest.