Pay off mortgage or purchase other assets?

[Jeffrey Boyce]

Hi there, I'm new to BiggerPockets but want to start leaning further into Real Estate to build assets. Currently I own 2 properties, a condo and home. 

Regarding the condo, I've been debating whether or not I should pay off the mortgage entirely or use those liquid funds to invest into other assets (stocks, Index funds, IRAs, etc). If I pay off the mortgage, I would gain about 20% of it back annually from rent. There are so many different scenarios and investment strategies that run through my head so I thought I'd get opinions from others. Thanks!