What's the difference between social security and fund investment? Which is more cost-effective?

These are two completely different concepts. The social security fund is for the future retirement pension security fund, so in the case of the ability to best adhere to the payment

Fixed investment fund is an investment investment behavior, is an investment will have risk.

The fixed investment fund is a monthly long-term unused idle funds, can be used to fixed investment, so that there can be amortized low investment costs, reduce investment risk, so as to achieve the purpose of obtaining a better return.

So it's best to pay the pension fund in the first place, the extra money and then choose to invest in fixed investment funds as well.