Contributions attract relief at your marginal rate subject to annual and lifetime-style allowances, and investments grow free of UK income and capital gains tax. Withdrawals after the minimum pension age are generally partly tax free with the balance taxed as income.
The trade-off is access. Money cannot normally be taken before the minimum age, which is scheduled to rise, so a SIPP suits long-horizon capital rather than trading capital.
Investment freedom is wider than a stocks-and-shares-isa, covering commercial property and a broad fund range, but leverage and speculative derivatives are generally restricted by the provider rather than encouraged.
General information about the United Kingdom, not tax advice. Rules change and depend on your circumstances; take advice from a qualified professional.
Related: stocks-and-shares-isa, uk-capital-gains-tax, hmrc, rrsp