Extra 50K better to pay down a Rental Mortgage or Re-invest

[Peter Schuyler]

This type of question may have been asked in the past.  

If I have 50K in cash to invest outside my retirement.

Is it better to pay down the liabilities on my rental properties that have mortgages at 4% or invest in another property that generates 8% cash on cash and use that money to pay down all the debt, including the new mortgage it will have?

I think I know the answer, but curious what others might do with the 50K.  Some say accelerate paydown, others say debt is cheap, so buy another property with the 50K as down payment, borrow the difference and keep going.