The bottom line: Unless you are on the verge of retiring and know your income will drop, the Roth wins.

3. Lease a car or buy a car?
Buying costs less if you own your car till it drops.

With leasing, you always drive a shiny new car, and your monthly payments are lower. So why would you buy?

Strictly by the numbers: A 2008 Toyota Camry will run you just under $27,000 (including taxes and fees). Buy one and finance it with a no-money-down, five-year loan at 6.9 percent (today's average), and your monthly payments will be $526. Over five years, you'll spend $31,560. Say you instead pay $1,000 up front for a five-year lease. Your monthly payment will be $415. At the end of five years, you'll have spent $25,900.

So leasing wins? That's not the full story. If you buy a car, it'll be worth something once you've paid off the loan. Your Camry would fetch about $10,000 after five years, according to estimates by Edmunds.com. That cuts the out-of-pocket cost to $21,560.

But wait: What if you really want a new car? In that case, the gap narrows. Take out a three-year loan on the Camry and your monthly payments would spike to $821, for a total of $29,556. If your three-year-old Camry sells for $14,000, your net cost drops to $15,556. If you leased, though, your payment would be $485 a month, putting an extra $336 in your pocket compared with the loan. Invest that $336 in a money-market fund paying 4.5 percent, and you'd earn $589 after taxes in three years, assuming a 28 percent tax bracket. Factoring that in, your total lease cost would be $17,871. Also, the Camry holds its value well. With models that don't, the manufacturer often sweetens the deal by inflating the car's assumed value at the end of the lease term. If you buy the same car, you won't make that much when you sell it, which could tilt the scales in favor of leasing.

You do the math: Use the calculator at Edmunds.com.

Beyond the math: Leases come with mileage restrictions, typically 12,000 miles a year. Plus, if you ding a leased car, you'll get dinged with fees.

The bottom line: Over the long term, buying costs you less.

4. Prepay your mortgage or invest?
The feel-good choice isn't necessarily the smart choice.

When some extra cash comes your way, it's tempting to put it toward your mortgage. You'll save on interest and pay off your house earlier. Buying stocks, on the other hand, feels like a risky leap into the unknown, especially now.

Strictly by the numbers: Paying off your mortgage or any loan is an investment, and your return is essentially the interest rate on the loan. If you have 25 years left on a 30-year mortgage with a fixed rate of 6.2 percent and you deduct your interest payments on your taxes, you'll earn 4.5 percent by prepaying the loan (assuming you're in the 28 percent tax bracket).

Now let's say you invest your spare cash in stocks instead. You'll pay a 15 percent tax rate on your long-term capital gains and dividends. So to beat the 4.5 percent return you'd get from prepaying your mortgage, you'd have to earn just 5.3 percent a year on your stocks before taxes.

The odds of your doing that over the 25-year remaining term of your mortgage are excellent: Historically, a portfolio of 80 percent stocks and 20 percent bonds has returned 7.5 percent a year after taxes.

But wait: Paying down the mortgage earns you a risk-free 4.5 percent. That's as good as you'll do with Treasury bonds. True, and if you are investing for a near-term goal and don't want to take any risk, you can make a stronger case for prepaying your mortgage. But if you are investing for a goal that's more than a decade away, you can and should take more risk for a chance at a higher return.

You do the math: To run the numbers on how much money you could end up with by investing, use the savings calculator at CNNMoney.com. To see how much interest you'd save by prepaying your mortgage, use the payoff calculator at Dinkytown.net.

Beyond the math: Of course, all that mortgage debt may be keeping you awake at night, especially if you are worried about losing your job or you're approaching retirement and hope to live on less. You'd be grateful to be rid of that major monthly bill sooner. In that case, prepaying your mortgage starts looking better. Remember, though, that by prepaying your mortgage, you are reducing your liquid assets. If you suddenly need money, it's easier to sell a mutual fund than it is to pull cash from your home, and you can always pay off your mortgage later with the money you invest now.

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