When calculating tax savings from mortgage interest, should you use your marginal tax rate or average tax rate

The TurboTax and HR Block quick calulators appear to use average tax rate to determine tax savings. Our average tax rate was 15% this year. Most other calculators specifically designed for calculating mortgage interest tax savings use marginal tax rate, which for us is 25%. Which are we supposed to use for an accurate estimation [...] http://bit.ly/muttIe