pay down my current mortgage first or invest?

[Gina Chang]

Hello, I need to hear your opinions on this. My situation is: I have a steady job and a steady income which allows me to pay my current mortage on a biweekly basis but I still have 15 years to go pay off my current mortgage. If I will have an extra $50k-60k cash income every year for some years, should I use the money to pay down my current mortgage first or should I start investing rental properties? My current mortgage rate for my home is 4.375. Thank you for your input in advance.