The core trade-off
The Public Provident Fund (PPF) is a government-backed savings scheme with a fixed 15-year term, currently earning 7.1% p.a. (revised quarterly by the Ministry of Finance), fully guaranteed with no market risk. A SIP invests in mutual funds whose return depends entirely on market performance — potentially much higher over the long run, but with no floor and no lock-in forcing you to stay invested.
PPF also carries the strongest tax treatment available in India: contributions qualify for Section 80C deduction, the interest earned is tax-free, and the maturity amount is tax-free too (this "Exempt-Exempt-Exempt," or EEE, status). Equity mutual fund gains are taxed on redemption (12.5% long-term capital gains above a ₹1,25,000 exemption per year).
A worked comparison
Investing the PPF maximum of ₹1,50,000 a year (₹12,500/month) for the full 15-year term: PPF at its current 7.1% p.a. grows to about ₹40,68,209, on ₹22,50,000 invested — a guaranteed ₹18,18,209 in interest, entirely tax-free. A SIP of the same ₹12,500/month for 15 years, assumed to return 12% p.a., projects to about ₹63,07,200 — roughly ₹22,39,000 more, but that gap exists only if the assumed 12% actually plays out over the full 15 years, and it's pre-tax (equity LTCG tax would apply on withdrawal).
The comparison looks very different over a shorter window — PPF's guaranteed rate is immune to market timing, while a SIP redeemed during a market downturn could show a much smaller gap, or even underperform PPF, depending on when you need the money.
Lock-in and liquidity
PPF has a hard 15-year lock-in (extendable afterward in 5-year blocks), with limited partial withdrawals allowed from the 7th year onward. A SIP in an open-ended mutual fund can be redeemed at any time with no lock-in (except specific products like ELSS, which has its own 3-year lock-in) — a meaningful advantage if your goals or circumstances might change.
A practical way to decide
PPF suits money you're comfortable locking away for the long term and want protected from market swings and from your own temptation to withdraw early — many people use it specifically for retirement, alongside EPF. A SIP suits goals where you want the possibility of a higher return and value the flexibility to redeem early if plans change. Since PPF's ₹1,50,000/year cap is well below what many investors want to save, most long-term investors run both — maxing out PPF for its guaranteed, tax-free base, and directing additional savings to SIPs for growth potential.