So you think you're ready to roll the dice on a unique mortgage loan? Then take a look at this analysis of a Fidelity Investments/GMAC Mortgage deal. It shows just how complicated evaluating options can be when multiple variables come into play -- one reason consumer advocates point unsophisticated borrowers toward plain-vanilla, 30-year fixed-rate mortgages.

Under the conditions spelled out below, a borrower who goes with the Fidelity loan can do OK if the portfolio backing the mortgage performs well. But somebody who converts assets to cash for a down payment will do better, assuming the money saved on monthly payments is invested rather than spent.

So many assumptions go into the analysis that it's hard to compare this to what a Fidelity borrower might actually experience.

We assume an 8 percent rate of return on the investment portfolio, for instance, but there's no guarantee stock or bond markets will actually perform at that level. We also used the 7.125 percent mortgage rate GMAC offered on May 27 in figuring out the payments.

One other thing: we didn't look at the tax implications of each borrower's choice. For instance, the holder of the smaller mortgage would pay capital gains taxes on the asset liquidation, reducing his net gain. The second borrower, meanwhile, would get a larger tax deduction each year because he would be paying more mortgage interest than the first, boosting his bottom line return.

As you can see, there's a reason experts advise borrowers not to get in over their heads, either at the casino or the lender's office.