On contrary if you count 80c tax benefit a person paying tax at 10% gets interest of 18%.8000 on 1 lakh investment plus 10000 rs saved as tax total 18000 on 1 lakh.Plus in long term compounding powers multuply returns with tax free return.
But as said RD is different and shouldnt be compared with ppf.
RD is done with 2-3 years horizon ad ppf is very long investment.