I'm going to use fake, round numbers here to make this easier.
If I have $15k, should I apply it to the principal of a loan that is at 5% or invest it knowing that the investment will always provide exactly a 5% return each year? This seems like straight math, but something tells me it is not.
More realistic question:
If I have $15k, should I pay off my car loan and trailer loan, assume both are around 5%, or invest the money?
What about applying it to my mortgage instead, which is around 3%? We take the standard deduction on our taxes, if that matters.