Traditional and Roth 401(k)s hold the same investments and share the same contribution limit. The only real difference is when you pay tax — now or later. For high earners, that single question is worth getting right.
Pre-tax (traditional) wins if your tax rate in retirement will be lower than it is today. Roth wins if it will be higher. Since nobody knows future tax rates for certain, many high earners deliberately split contributions to hedge both ways.
Traditional (pre-tax): a deduction now
Contributions reduce your taxable income today, and you pay ordinary income tax on withdrawals in retirement. For someone in a high bracket now who expects to be in a lower bracket later, that upfront deduction is valuable — you defer tax at a high rate and (ideally) pay it at a lower one.
Roth: tax-free later
Roth contributions are made with after-tax dollars, so there is no deduction today — but qualified withdrawals in retirement, including all the growth, are tax-free. Roth wins if you expect higher rates later, and it adds valuable flexibility: tax-free income can keep you under thresholds that affect Medicare premiums and how your Social Security is taxed.
Why "both" is often the high earner's answer
Future tax law is unknowable, and an allocation of pre-tax and Roth money gives you choices in retirement — you can draw from whichever bucket is most tax-efficient in a given year. A common approach is to lean pre-tax while your income (and bracket) peaks, and direct some contributions to Roth to build a tax-free reserve.
One thing that does not change with this decision: how the money is actually invested. The tax treatment is separate from your allocation — and your allocation is what drives long-term growth.
For the investment side of the decision, start with 401(k) allocation by age. And if you are weighing a Roth 401(k) against a Roth IRA, see Roth IRA vs. 401(k).
The part that actually matters: discipline
A sound allocation only helps if you hold it through the rough stretches and change it for the right reasons. That is the idea behind the 401(k) Edge strategy — a rules-based monthly update that works with any plan and any provider, so your 401(k) follows evidence instead of emotion. Most months, it says do nothing.