Find one good management company for all your properties.Diversify -- buying in different areas, use multiple management companies.I think for a large portfolio (20+ properties), diversification will help stabilize the investment.

To me it is a matter of risk diversification. 

Really comes down to diversification in an economy.

Commonly called a diversification exchange you can divide the cash between as many properties as you want as long as you meet the two prong rule of purchasing at least as much as you sell and using all the cash to do so.It is not uncommon for an investor to purchase one property with the minimum down and another property with the remaining excess cash down so they can refinance that property after the fact and take cash out.

Lots of real estate market diversification, so probably a good thing.

Ideally, I would rather buy two $150K homes for diversification purposes but this question is something I've been wondering about: if I choose the $150K option then the bank might not let me finance another $150K house for at least a year... 

I also believe in diversification across different asset categories...I'm not a fan of all of my eggs in one basket...good, bad or otherwise. 

Are you seeking long-term appreciation, rental income, or diversification?

I've always believed in diversification but am not really willing to put in the hours learning a new market(s).

I now have properties in a few neighborhoods and see it as a bit of diversification.