I have kind of gone back and forth on this myself. I prepaid my mortgage (5.375% rate on a rental property) but stopped after about $15k paid down. I only have about $40k left, which I could wipe out in less than 2 years and be mortgage free at 33. If I paid it off I would lower my DTI allowing me to more easily qualify for other investment properties, for cheaper. I don't make much (even with the rental income included) and so it doesn't take much debt to put me out of reach of the loans I would need to get this rolling.
Obviously I am investing in real estate because I am seeking to beat the market with rental properties. I currently only have retirement in stocks, which I contribute up to the employer max (since age 25), but never more, and some cash in a ROTH I do not currently contribute to.
For now I think I will hold off on paying off the mortgage on my one rental, and instead recast it as I believe that will reduce the monthly payment enough to give me a qualifying DTI. I plan on using the cash I would have used to pay it off, to purchase a second rental early next year. I do want to pay off property 1 before buying a 3rd property as it used to be my primary property, and I plan on making it my primary property again one day, and I don't want it to be at risk of foreclosure if something should happen. Since I don't have any stock investments that don't come with a penalty, I don't have an incentive to turn to my investments to pay my mortgage in a time of crisis; I would be paying a steep price to do that.
I do want to start investing in mutual, because I believe in a well-rounded portfolio (that is the main reason I don't want to automatically pour all of my money into stocks). I just want to first get over the hurdle of buying at least a second and 3rd property, so I can use the cashflow from them to run my real estate game, while I can use my earned income for investments. I highly expect someone to tell me I am doing it all wrong LOL.