An overlooked but brilliant way to build retirement wealth is a Self-Invested Personal Pension, or SIPP. While all ISA withdrawals are free of tax, the SIPP scores by offering pensions tax relief right at the beginning, when you pay money in. Further withdrawals will be added to your total earnings for that year, and subject to income tax. At The Twelfth Magpie, we favour investing in a SIPP or ISA, or both, by building a balanced portfolio of FTSE 100 and FTSE 250 shares. Why I love this FTSE 100 stockI added wealth manager M&G (LSE: MNG) to my own SIPP three years ago and it's been a terrific investment.