If you have it in cash, you can make it work for you in other ways, either by buying another house, or putting it in the stock market.If you have no desire to grow your portfolio further, then paying off the loan earlier can makes sense.

Or hang on to 1st floor and maintain month to month while I rehab second floor?

I don't look for association maintained properties at all locally.  

As the stock market has gotten better in the past three months, the 10yr treasury yield has increased as well.

So they can't really "shop around for a better price" and maintain that neutrality.

It's been empty (but yard maintained) with no attempt to sell for over a year.

You should be taking the 50k out to beat the returns of the silly stock market by studying real estate.

What I would like to know, is if there is a forum that is just like this one, but is instead dedicated to investing in the market, like stocks, mutual funds, etc.

We invested exclusively in the stock market until last year. 

I am happy with a 6-8% ROI each year on a cash deal rather than having it sit in the bank or the stock market.