Same money out of your pocket, which leaves you more to actually spend after tax in retirement? Drag the inputs and compare.
Both options start from the same amount out of your pocket. The ISA invests that money from your taxed income; it grows and comes out completely tax-free. The SIPP grosses your money up by your tax relief (so more is invested), grows tax-free, then on the way out 25% is tax-free and the rest is taxed as income.
So the SIPP wins whenever your relief in plus the tax-free lump sum beats your tax on withdrawal, which is most of the time, and dramatically so if you pay 40% now but 20% in retirement. The ISA only wins on tax if you'd draw it at a higher rate than you saved. The ISA's real edge is access: no lock-in.
Sources: gov.uk & HMRC (pension tax relief, tax-free lump sum, 2026/27). Illustrative; assumes constant return and tax rates, relief reinvested, and withdrawals staying within the chosen rate band.
Illustrative only, not financial or tax advice. Assumes relief at source fully reinvested, constant returns and tax rates, the 25% tax-free lump sum, and England/Wales/NI rates. Real outcomes depend on contribution limits (£20,000 ISA / £60,000 pension), how you draw the pension, other income, and benefits. Pension money is locked until 55 (57 from 2028). Always do your own research or speak to a qualified adviser.
Pairs with Pension Tax Relief and Compound Growth.