Should I pay January 2018's mortgage before January 1st 2018 or wait? which would be better under the new tax law?

Starting in 2018, the deduction on a first mortgage will be limited to $750,000 of loan balance, but the current limit of $1,000,000 will still apply to mortgages that were in place before Dec 15, 2017.

Interest on home equity debt will not be deductible at all, currently it is deductible up to $100,000 of debt.

The standard deduction is doubling from $12,600 to $24,000 for married filing jointly.

The marginal tax rates are going down by 2-3% across the board.

So, if your current total of all itemized deductions (schedule A) is more than $12,600 but less than $24,000, you will use the standard deduction next year, so it would save some tax money if you pre-paid next year's mortgage interest in 2017.  Or, if you have home equity debt, it would benefit you to pre-pay that interest.  (**In both cases, make sure the bank will credit your payment to interest instead of principle.  You may want to call them first.)

If neither of those situations applies, then the only effect of prepaying to shift your deduction from next year to this year is a 2-3% larger savings.  (Pre-paying $1000 of interest would save $20 in tax over time.)