Quite honestly, there is money to be made in every market if you buy correctly. If you're looking for natural appreciation and some cash flow, use the strategy that your friends have suggested. With that strategy, however, you will probably miss out on forced appreciation (equity creation) and some bigger cash flow numbers because of the premium you would have to pay to buy in those areas.
More importantly, the best places to invest are the places you're familiar with and areas that you have a team in place. Wherever you decide to invest, make sure you have trusted lenders, insurance agents, attorneys and MOST importantly great property management. Property management will make or break any RE investments you make.
In terms of metrics, look for population growth, job growth (2% over the last 12 months), low crime rates (Trulia crime maps). If you're that interested, this is a great resource (udemy.com/realfocus).
Good luck and feel free to reach out with questions!