You've got a moving target, hopefully you are good with Excel or you know a good accountant.
Basically, do you think interest rates will go up or down? Will Pres. Trump be good or bad for the economy? If you think interest rates will go down, you should use the short term rate and adjust it each year, rather than be locked into a higher interest rate.
Just make sure that everything you do is in writing, you need a contract with your mother that specifies the interest rate and how it will be calculated or determined (such as, the short term AFR on Feb 1 of each year). You probably should not put the gift in the contract because gifts are not enforceable in most cases. (A contract is an agreement to give something in return for getting something, and can be enforced on that basis. A free will gift usually can't be enforced.)
And whatever interest you pay is taxable income for your mother. Even if she doesn't actually charge interest (because she gives it back to you) she must report it as income.
