Would you hold a $1,000/month cash flow property (before OPEX) or sell for 120,000 profit? What factors would you consider when analyzing?

Assuming this is a class B or C property (can't imagine a property in a nice area would cashflow like that), I'd hold onto it for the cashflow so long as the neighborhood wasn't depreciating or there was a net population outflow. That being said the big factor for the calculation is the equity in the property.. the more % equity you have, the less leveraged you can be on that $120k profit, reducing your IRR.

What's the Property Value of the house right now?

What can you buy for $120k DP/$600k PV in the way of CF?

In other words, can you reinvest the $120k and get higher than $600k PV and $100/M CF? If the answer is yes, and I'm guessing it is, then sell.

I'd Refinance it if you have that much equity. Best of both words. Get your cash out, and still have some cash flow. BRRRR principles. Obviously, more numbers to analyze but with the given numbers this would likely have a high probability of earning the highest ROI.

ROI is arguably the most important variable... as it determines the rate of compounding your money...which compounding your money at the fastest rate, creates the most wealth.

What could you do with the equity and what return would that give you?  Run the numbers if you refinance, if you sold it and bought another rental (or two) or if you held it for another 5 years.  For each of these include all of the costs such as refinancing costs, commission and taxes if you sold it, etc.