Use cash flow to pay off my SFR rental or invest in the S&P 500?

[Matt Wells]

Hello, I have around $550 a month positive cash flow from a couple of SFR rental properties. I'm debating on whether to use the positive cash flow to pay off the SFR principal or to invest it in a Vanguard S&P 500 mutual fund. It would be helpful to hear some pros and cons of each.