@Prakash Singh I'm definitely not an economist but in general the more money that is in our economy the higher the chance of inflation. Currently the Fed's goal is an average of 2% inflation per year. They pump money into the economy to try to push up inflation. If a widget today costs one dollar today then next year with 2% inflation the same widget costs $1.02. In 5 yrs it will cost you $1.10 for the same thing. Your income is the like that one dollar widget. 5 years from now you are getting 10% more but your cost of the loan didn't change with a 30 year fixed rate mortgage. So you have more money to make your payment with.
The downside of inflation is when you are holding on to cash. It is worth less everyday. That's why you will hear Robert Kiyosaki say "Savers are losing." Right now the bank is paying you half of a percent or less on your money you have on deposit. If inflation is running at 2% like the Fed wants than you are losing 1.5% on your money every year.
All things being equal I would take rental income and put it into the next rental rather than paying down your mortgage faster or saving it in the bank. Just my thought.