Model the after-tax wealth of each account at retirement and find your personal break-even tax rate.
Both accounts shelter investment growth from annual taxation, but they differ in when you pay tax. With an RRSP, you deduct the contribution from income today (getting a refund at your current marginal rate), and pay tax on every dollar withdrawn in retirement. With a TFSA, you contribute after-tax dollars and pay nothing on growth or withdrawals, ever.
The math is elegant: if your marginal tax rate is identical now and at retirement, both accounts produce exactly the same after-tax wealth. RRSP wins when your current rate is higher than your retirement rate; TFSA wins in the opposite case. The break-even rate printed above tells you the retirement tax rate at which the two accounts tie.
In practice, high earners in their peak working years typically benefit more from RRSP (large deduction at a high rate, withdrawal at a lower rate in retirement). Lower-income earners, young savers, and anyone worried about OAS clawbacks or GIS eligibility often prefer TFSA. Many Canadians contribute to both.