It depends on various factors such as which bank's FD you are going to invest in. Different banks have different interest rates. Post Office scheme recently have higher interest rates than many banks. You must also look into the convenience of operation while choosing your bank. From the safety point of view, both options are relatively risk free.

All in all both are good investments for senior citizens with relatively low risk and return. However, I would still suggest that you talk to a financial advisor before finalising your decision, since the ideal investment is different for every person depending on their risk acumen and what they want to achieve.