It's strictly based upon the interest rate that you currently pay on the mortgage versus the return that you can get on the IRA investment.

Your mortgage interest is tax deductible, however, so a 6% interest rate might equate to an actual rate of something less than 5% depending on your tax bracket.

If you can get more than 5%, consistently, in the IRA investment, then, it's a better choice. But, there is some risk in the IRA, I presume. So, the rate of return must be somewhat higher than 5% to compensate for this risk. You need to evaluate the level of risk dependent on the investment.