With interest rates low, more homeowners have been opting for a 15-year fixed-rate mortgage over the traditional 30-year mortgage, both for new purchases and to refinance existing loans.

Although the monthly payments are higher, they can save tens of thousands of dollars in overall interest costs, plus own their homes free and clear in half the usual time. Also, 15-year mortgages typically carry lower rates - up to half a point lower.While that's mighty appealing, some financial experts argue it may not be the best route, even for those who can easily afford an accelerated payback.

"Many homebuyers . . . would be much better off taking the lower 30-year payment and investing the difference in a tax-deferred investment plan," such as an employer-sponsored 401 plan or Individual Retirement Account, concluded a study in the April edition of the Journal of Financial Planning, trade publication of the Institute of Certified Financial Planners.

The study, conducted by Delbert C. Goff and Don R. Cox, professors from Appalachian State University in Boone, N.C., analyzed a $150,000 mortgage for a borrower in the 33 percent combined federal and state tax bracket. The borrower has to choose between a 15-year mortgage at 7.5 percent and a 30-year loan at 8 percent. (Current market rates are actually lower - around 6.8 percent and 7.2 percent, respectively.)

Under the 15-year mortgage scenario, the borrower begins a savings plan only after the mortgage is paid off, then invests the entire mortgage payment in a tax-deferred retirement plan earning 10 percent a year for 15 years.

The borrower with a 30-year loan, on the other hand, immediately invests in a retirement plan, also yielding 10 percent, the monthly difference between the higher payments of the 15 year and lower payments of the 30 year.

(The monthly payment on the 15-year loan amounts to $1,390.52, or $16,686 annually, excluding property taxes and insurance, and on the 30-year, $1,100.66, or $13,208 annually.)

The study found: The home owner with the 30-year mortgage builds a retirement account of $1.17 million vs. $791,000 for the owner paying off the mortgage in 15 years.

While most financial experts agree the plan works well in theory, some question whether it can be carried out by average homeowners.

"It assumes the borrower will have the discipline to set aside monthly savings and invest each month. Look at the dismal savings rate for the U.S. vs. other countries," said Peter G. Miller, a Washington, D.C., author of several consumer real estate books. "The assumption is also that investing each month will yield a higher rate of return than investing in your mortgage."

Jonathan Pond, a Boston-based financial planner, agreed: "The assumption is that all of your money is put into stocks. But most people prefer to have both high- and lower-risk investments, which include bonds, which have a lower rate of return."

Pond, however, thinks borrowers will be better off in the long run contributing to a retirement plan than making an extra mortgage payment. If they can do both, all the better.

"It's a wonderful feeling paying off a mortgage early," he said.

One rule of thumb for determining whether to go for a 15 year vs. a 30 year: "Do not take a 15 year out in lieu of a 30 year if you can't comfortably max out on a retirement savings plan," and make the maximum contributions allowed by an employer.

Right now, roughly 15 percent of all borrowers choose a 15-year mortgage vs. 69 percent for the 30-year and 12 percent for adjustable-rate mortgages, according to the Mortgage Bankers Association. Even higher percentages have switched from a 30-year to a 15-year loan in this low-interest rate environment.

Miller, who also operates a real estate web site on the Internet (www.ourbroker.com) thinks most home owners would be better off with a 30-year loan, mainly because the payments are lower. He says borrowers can easily shorten the life of their loan on their own simply by adding, even nominally, to their regular monthly payments. That's also provided there's no prepayment penalty.

An extra $50 a month, for example, would cut 4 1/2 years from that same 30-year $150,000 mortgage mentioned in the financial planners' study. The borrower would save $44,000 in interest as well, he said.

This method, Miller says, is more flexible than taking on a biweekly payment schedule in which borrowers agree to make 26 half-payments, which add up to 13 full payments a year. It's the 13th payment that shortens a fixed-rate loan from 30 years to about 21. One drawback to that arrangement is that it's usually done through a third party, who charges a fee.

The Appalachian State University study says the 30-year strategy works best when the buyer is in a high tax bracket, owns the home for a long period of time and when rates are low.

Whatever mortgage decision is made, however, the study reminds home buyers that they should take their overall financial plan into account when deciding on a mortgage.