The only rule that really matters
If your marginal 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-career in a low bracket), Roth wins. The accounts are mathematically identical if your tax rate is the same in both periods, so the entire 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 tax bracket — is usually at its lowest. Paying tax now at, say, 12–22% to lock in decades of tax-free growth is often a bargain versus deferring into an unknown (and possibly higher) future rate. Roth also has no required minimum distributions, gives tax-free flexibility in retirement, and contributions (not earnings) can be withdrawn penalty-free in a pinch.
When Traditional is the smarter call
High earners in their peak years often benefit more from the immediate 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 thresholds for other benefits. For 2026, Roth IRA eligibility phases out between $153,000–$168,000 (single) and $242,000–$252,000 (married filing jointly); above that, a Traditional or backdoor Roth is the route.