The only rule that really matters
If your top tax rate in retirement will be lower than today, Traditional wins. You deduct at a high rate now and pay at a low rate later. If your retirement rate will be higher (or you're early in your career in a low bracket), Roth wins. The two accounts work out exactly the same if your tax rate is the same in both periods. So the whole edge comes from the rate difference, not from one account being magically better.
Why Roth is the default for younger savers
Early in your career your income, and your tax bracket, is usually at its lowest. Paying tax now at, say, 12% to 22% to lock in decades of tax-free growth is often a bargain versus putting it off to an unknown (and possibly higher) future rate. Roth also has no required minimum distributions, gives tax-free flexibility in retirement, and you can withdraw your contributions (not the earnings) without penalty if you ever need to.
When Traditional is the smarter call
High earners in their peak years often gain more from the upfront deduction, especially if they expect to retire in a lower bracket or in a no-income-tax state. The deduction also lowers this year's taxable income, which can keep you under the limits for other benefits. For 2026, Roth IRA eligibility phases out between $153,000 and $168,000 (single) and $242,000 and $252,000 (married filing jointly). Above that, a Traditional or backdoor Roth is the route.