The standard advice you’ll hear from most “experts” is that the type of IRA you should open depends on whether you think the tax bracket you’ll be in when you retire will be higher or lower than the one you’re in now. If you think you’ll be in a higher tax bracket, you should choose a nondeductible Roth IRA. If you think you’ll be in a lower tax bracket, you should choose a traditional deductible IRA. (Pretty much everyone agrees that a nondeductible traditional IRA makes sense only if you aren’t eligible for any of the other IRA options.)

This may seem simple enough, but predicting your tax bracket isn’t that easy. Most people assume they’ll be in a lower bracket because they won’t be working anymore. But how safe is that assumption? Surveys show that slightly more than half of all baby boomers—that giant generation that’s currently reaching retirement age—do not plan to quit their jobs when they hit 65. In fact, three out of four current retirees still do some sort of work. And in any case, don’t forget that your Social Security benefits are taxable. So your taxable income may be higher than you think. What’s more, who knows what tax rates will be when you retire? It’s certainly conceivable that they could be much higher than they are now—meaning that even if you were earning less, you wouldn’t necessarily be in a lower bracket.

So how do you decide? Well, one thing you can predict with a fair degree of certainty is how long your money will have a chance to compound in your IRA. This is an important thing to know, because it happens to be a fact that, the longer the time frame, the better off you’re likely to be with a Roth IRA. That’s because, with enough time to do its magic, the miracle of compound interest will put you so far ahead that not having to pay taxes on your withdrawals after you retire is bound to be worth a lot more to you than being able to deduct your contributions now.

So if you’re relatively young (say, under 35) and have decent prospects, a Roth is generally the way to go. If you’re between 35 and 50, your decision should depend mainly on how badly you could use the $1,000 or so in tax savings you’d realize from the up-front deduction that a traditional IRA lets you take. If you could live without the money (and you meet the income requirements), you’d probably be better off making your contribution to a nondeductible Roth and reaping your rewards down the road. But if you’re 50 or over, the traditional deductible IRA is almost always a better deal.