(MONEY Magazine) – Q. My employer is offering a Roth 401(k ) in addition to our regular plan this year. Where should I put my money? —Jeff Kay, Roselle, Ill.

A. Would you prefer to pay taxes on your retirement savings now or later? That's the central issue. With the Roth 401(k), as with the Roth IRA, you fund the account using money that's already been taxed, so you won't owe Uncle Sam a dime when you make withdrawals. Traditional 401(k)s give you the tax break up front, and then you pay the feds at your regular income tax rate when you withdraw money. If you could be sure of your tax bracket in retirement, the answer would be easy: higher bracket later, fund the Roth now; lower bracket later, go traditional now. But unless you've got a pretty good crystal ball, you have no way of knowing definitively. That's why planner Chip Simon suggests hedging your bets: Fund the Roth when you're younger and probably earn less (which means your bracket is likely lower now); then, as your fortunes improve in mid-career, switch to the traditional version.