If you are a long term buy and hold investor, I would look at where the growth is happening....buy in the areas of high growth....as Ross Perot used to say, you want to make a billion in real estate, buy in the path of progress....and he and now his family did quite well with this idea.Austin prices are super high, but seem to be going higher. 

Some of the biggest growth is south side of Corpus.

(How are they measuring rent growth, is it greater than appreciation?

Indy is similar to Columbus with prices and growth but I personally like Columbus better for its growth with new companies moving here and its tech space rapidly growing.

When   choosing where to invest, I’ve read and listened that things to consider are population growth, job growth, etc are important.

You'll be able to accelerate your investment growth in a particular area if you hyper focus on it.

Given the amount of equity and what you are currently cash flowing, I would probably take some cash out, find a property in a landlord friendly/growth market, and build my portfolio. 

The equity growth would be nice, though. 

Question is how small is too small that would effect the future rental growth and property value?  

If you viewing every task/hour that one partner is doing and trying to find a perfect match up, then perhaps its not a good partnership, or you just have a dynamic that puts a dollar value to every bit of effort.Good luck.