Diversification is the best economy along with population growth.  

Make sure you concentrate on the economy and the population growth when you make a choice and stay out of areas you do not know.  

Also, it doesn't work well in all areas of the country, that rule works well in rural areas or in large parts of the South & Midwest, but it doesn't usually work in more densely populated areas along the coasts like the Philadelphia metro.Before deciding what part of Philly you want to invest in, you should decide whether you want to manage the property yourself, or whether you want a property management firm to take care of it.

I have been looking at job growth, population and income in the cities. 

Relevant to you and your tenant:If you just took his monthly rent number, cut it in half, and collect it along with his biweekly pay, you'd end up collecting 13 months of rent per year, not 12.I'm not suggesting you're cheating anyone, just throwing it out there that someone might be ACCIDENTALLY over paying or over charging, because 95% of our population thinks there are four weeks in a month (ok 95% of people, why do we have 52 weeks a year and not 12*4 = 48 weeks per year?

But, we do have pretty strong job populations as well...

I say this because less than 3% of the population can really afford to self-insure long term.

It's incredibly tough to see rents fall if home prices are decreasing unless the population of the area in which you have the rental is decreasing.  

Be in a diverse economy and have a growing population.  

Dallas of course has a booming economy and growing population but the property taxes and insurance kill your cash flow.