I think much of the decision to sell or not depends on 2 things. Current cashflow and future potential of the area.
Well 3, the 3rd is how you feel about having rentals and the occasional issues that come up.
Taking into account all expenses and potential expenses (Property management, vacancy, maintenance, repair, and capital expenses-like a roof), do you make money?
Future potential of the area - What will it be worth in 20 years? Are there jobs in the area and potential for more, is there limited land for development? Those are the 2 things that mainly determine value and growth. You said it's a good area and there's development, so you are partway there.
My personal experience is that if I have a rental that I like in an area I think has future potential it's much easier to ride out the issues that come up and down times.
You are young enough to take a long term approach on this if thats what you want to do.