The core trade-off
A Fixed Deposit locks in a known interest rate for a fixed term — you know exactly what you'll get back on day one, and it's protected (up to ₹5 lakh per bank) under India's deposit insurance scheme. A SIP (Systematic Investment Plan) invests in mutual funds, whose value moves with the market — you could end up with meaningfully more, or, over short periods, less than what you invested.
Neither is "better" in general — an FD is the right tool for money you'll need on a specific date and can't risk losing; a SIP is the right tool for a long-term goal (5+ years away) where you can ride out short-term market swings for a shot at a higher return.
A worked comparison
Investing ₹5,000 a month for 10 years: a Recurring Deposit at a typical 7% p.a. (compounded quarterly) grows to about ₹8,68,509, on ₹6,00,000 deposited — a guaranteed ₹2,68,509 in interest. A SIP in an equity fund assumed to return 12% p.a. projects to about ₹11,61,695 on the same ₹6,00,000 invested — roughly ₹2,93,000 more than the RD, but that 12% is an assumption, not a promise; actual equity returns vary year to year and can be negative in a bad year.
The SIP's assumed edge only shows up reliably over long holding periods. Over 1-2 years, equity markets can easily underperform an FD's guaranteed rate — the comparison above is specifically for a 10-year horizon, where the odds of equities beating fixed-income historically improve.
Liquidity and tax differences
FD interest is fully taxable at your income tax slab rate every year it's earned (or accrued, depending on how you report it), with TDS deducted by the bank above a threshold. Equity mutual fund gains are taxed only when you sell (redeem) units — long-term capital gains (holdings over 1 year) are taxed at 12.5% above a ₹1,25,000 exemption per year, generally lighter than most FD investors' slab rate.
FDs can usually be broken early with a small interest penalty. Open-ended mutual funds are also redeemable anytime (unlike ELSS or other lock-in products), though redeeming during a market downturn locks in whatever loss exists at that moment.
A practical way to decide
If your goal is less than 3 years away, or you genuinely cannot afford for the value to dip even temporarily, prefer an FD or RD. If your goal is 5+ years away and you can tolerate seeing the value fall in a bad year without panic-selling, a SIP has historically been the more effective vehicle. Many people use both — FDs/RDs for near-term and emergency needs, SIPs for long-term goals like retirement or a child's education.