@Brian H. It's all about opportunity cost. $150 cash flow per month is undoubtedly more than $102 per month, but how much of your own money are you using? Is there obvious risk between the two properties you described above?
As an example, lets say you put a downpayment and finance a property that is $200k(40k downpayment), which gives you a 10% ROI. If you could find a property that is $100k (20k downpayment), which gives you a 14% ROI, you could take the money you saved and put it towards another similar property and elevate your returns.
Now you also want to assess the security of your investment.
Following my example above, maybe the $200k property is located in a great area which will attract great tenants and support potential appreciation. This can mitigate your risk by avoiding more tenant turnover and having lower vacancy rates. The cheaper $100k property might be in a less appealing area with a lower quality of tenants, and high tenant turnover or missed rent payments could affect your ROI outlook.
Personally, regarding the situation above, I would take the deal that is a little less ROI, but gives you better security on your investment. Maybe in your case, both properties are comparable in terms of security of investment, but I wouldn't go for a 8% ROI unless the property was in a very good market where appreciation is expected. For that kind of ROI, you could stick your money into low risk stocks/bonds and see similar returns without having to do any work.