Low rates: down payment vs. invest

[Ryan Armstrong]

I'm looking at buy-and-hold (rent) and thinking about down payment structure. Assume that I am going to pay PMI either way (not looking to put more than 10% down at the moment). Given rates are so low right now, what's your opinion on putting a lower down payment on a property in order to maintain the difference in investments (average 6%+ per year in the long-term)?