The conventional wisdom is that a Roth 401(k), with tax-free withdrawals, makes more sense if you expect to be in a higher tax bracket in retirement. Urban's analysis shows that even if your tax bracket dips in retirement, the tax-free accumulation in the Roth may still make it a better deal.
That's particularly the case if you can afford to put in the maximum in a Roth 401(k) from early on in your career. Young workers are often in lower tax brackets, minimizing the immediate tax benefits of traditional 401(k)s. But they also may be living on tight budgets and be unable to contribute the maximum.
If unsure which one is best for you, and your employer offers both, you can split your contributions between a traditional and Roth 401(k) and/or change your selection each year, depending on your current tax bracket and circumstances.