Rentals vs Stock Market

[Alex Zaboli]

I know that this platform has experimental bias, but I've gotten solid advice pointing to maxing out tax advantage contributions (e.g. 401k which allows 19k in annual contributions) and then simply index fund investing. 

Having a long time horizon (40 years) until retirement age, what are the key factors to differentiate wealth building via real estate rentals vs stock market investing? 

To more personalize this topic, I like the concept of cash flow provided by rental property units, but I want to better understand why I would opt for that when I could take a perhaps more simple approach and dollar cost average in the stock market where I could reasonably expect around 10% over the long haul. I've heard that I should target 10% cash on cash return for real estate rental investing. It's worth noting that I have a stable and growing job that allows me to live comfortably off earned income, so I don't need the cash flow to live off of. I want to understand what real benefit(s) that extra cash affords me - is it mainly that I could build out a rental portfolio quicker? 

Thanks for your feedback!