Alarmed by rising long-term interest rates, many mortgage shoppers are racing to lock in new loans at today’s still-attractive rates.

Makes sense: A month or two from now, mortgage rates could well be higher, thanks to the trickle-down effect of economic recovery.

But once one decides to get a new mortgage ASAP, what type is best under today’s conditions  a traditional fixed-rate 30-year loan, a 15-year fixed loan or an adjustable-rate mortgage that can charge a different rate every year?

For most borrowers, the tried-and-true 30-year fixed mortgage is probably best. Here’s a rundown:

Thirty-year fixed

Nationwide, the average rate was 7.12 percent as of a week ago, according to HSH Inc., the Butler, N.J., tracking firm.

New borrowers may be sad they didn’t lock in last November, when rates bottomed at a remarkable 6.59 percent. But today’s rate is still extraordinarily low by historical standards. For every $1,000 borrowed, the monthly principal and interest payment is $6.73 at 7.12 percent, $6.38 at 6.59 percent. On a $100,000 mortgage, that a difference of only $35 a month.

Fifteen-year fixed

HSH says these averaged 6.61 percent nationally as of a week ago. That’s a tad less than the 30-year fixed rate, reducing the interest portion of the monthly payment.

But since the loan has to be paid off in 15 years instead of 30, the principal portion is larger: The monthly payment would be $8.77 for every $1,000 borrowed.

A 15-year mortgage is appealing because it saves the borrower 15 years’ of interest payments. Over the life of the loan, interest on every $1,000 borrowed at 6.61 percent would total $579. Pay for 30 years at 7.12 percent and interest would be $1,424. That’s $84,500 more for a $100,000 loan.

There’s a problem with a 15-year mortgage, though: For 15 years, you must pay more every month than you would with a 30-year loan  you’re stuck with it. That’s fine, but only if you can make the larger payment comfortably.

For many borrowers, there’s an appealing compromise  get the 30-year loan and make extra principal payments voluntarily. This way you can pay off the loan in 15 years, or even sooner. But if money gets tight or you see a better way to invest it, you don’t have to make the larger monthly payments that a 15-year loan would require.

True, you don’t get quite as good a rate with this approach. But the savings offered these days by the lower 15-year loan is minimal anyway. Paying for 15 years at the 30-year rate of 7.12 percent will cost you $630 in interest per $1,000 borrowed. You’d pay $578 with the 6.61 percent rate offered on 15-year loans.

That comes to $5,200 for every $100,000 borrowed. Of course, this extra interest would be tax-deductible, shaving the cost of going with the 30-year loan.

Adjustable-rate mortgages

These “ARM” loans offer the lowest initial rates, averaging 5.46 percent as of last week. But that’s not enough to justify the long-term risk. Typically, ARMs can go up (or down) as much as 2 percentage points a year or 6 points over the life of the loan. An ARM issued today could go as high as 7.46 percent a year from now, and as high as 11.46 percent someday in the future.

Why risk that if you can lock into 7.12 percent for 30 years with a fixed-rate mortgage?

ARMs pay off only if they offer much bigger savings up front  or if you don’t expect to keep the mortgage long enough to care about the risk of higher rates later.

The typical ARM is adjusted every year to a level 2.75 percentage points above the rate paid by the one-year Treasury bill. These bills are currently paying about 2.3 percent. That means older ARMs scheduled for adjustment now could charge as little as 5.05 percent, unless the caps on annual changes keep them higher.

ARMs look attractive from this perspective, but rates probably won’t be this low for long. Many experts expect the Federal Reserve to begin raising short-term rates later this year. And the fact is, one-year bills are almost never this low.