What you do with your gains from the sale doesn't matter. It's taxable income unless you qualify for the "lived in 2 of last 5 years" exclusion rule.
One possible advantage of paying off the mortgate on a remaining rental, is that in following years you "MAY" be able to use up any carry over losses, if you have any. There are pros and cons to that though. But the major "pro" is that the paid of rental property will have a significantly higher cash flow, once that mortgage payment goes away. Just make sure to put enough aside to pay the yearly property taxes and insurance.
I myself have 3 rentals and paid one off back in 2013 or 14. I had an immediate increase in cash flow of around $700 a month. All my carry forward losses were "used up" with my 2017 tax return. So now my rental properties overall do show an actual taxable profit every tax year starting with the 2017 return.
What I do now is bank the first three months rent to hold for yearly property taxes, property insurance, and income taxes. The remaining 9 months rent is mine to do with as a please. I also make quarterly tax payments from that first three months of rent in savings, on what I estimate the actual "taxable" rental income will be too. Then at the end of the year, any "excess" in that savings account goes toward paying off another rental early.
