Savings vs paying off mortgage

[Rachel Trubisky]

my husband and I bought our first investment property in December. We aren't sure what we should be doing with the money left over from the monthly mortgage. We plan on buying additional properties in the near future. We could have the property that is currently rented paid off in 3.5 years if we put all of the collected rent on the mortgage. 

In your experience and opinions what do you recommend....paying off the mortgage or putting the money to the side and using it as additional down payment to the next property?