Older Couple buy investment property cash or with mortgage?

[Vincent Donato]

I'm a new real estate investor and am currently on my first duplex rental property in Ohio. I've some what inspired my parents with the moves I've been making. Between their interest in what I'm doing and my parents fear of inflation and a USD crash they've become potential partners. They think they would rather have their money in a physical asset like real estate than a Roth account, and where better to start than have your young son do the grunt work for you. 

I'm going to put together a proposal of different properties and cashflow possibilities. My question is, do I lead them in a direction of just putting a nice down payment on a mortgage or buy the house cash in full. I'm in the Cleveland area so cashflow rental properties are more prevalent than anything that will significantly appreciate in value. I know this seems like an obvious answer, but being that my parents are in their late 60's, if they were to purchase a 300k house they may never see the return, rather than if they took out a mortgage they would see that monthly cashflow to live off of. 

Interested to hear all ideas!