Fairfax City is a great place to buy if you like ...
Both of these answers are true depending on how you finish the sentences above. An agent can provide unbiased expert opinion if you want a more specific answer.
In Fairchester, I often go into homes that have not been updated, but there are some homes that have been beautifully updated. You need to see each home and see what the owners have done. For instance I have a home listed in Fairchester currently where the owners have replaced the kitchen and both bathrooms in the past couple of years. Those are major expenses that the new owner will not have to take on. The big advantage to a neighborhood like Fairchester is the size of the yards. Older neighborhoods were built with more space around them giving the whole neighborhood a different feel.
There are only two homes for sale in Fairchester right now. The link below will show them both to you. There are also other similar neighborhoods around Fairfax City (Fairview, Country Club Hills, Warren Woods, Fairfax Acres, Mosby Woods). Let me know if you want to see what homes are available in those neighborhoods.
Additionally, though the taxes are slightly less in the city, the services are higher. For instance trash service is free for all city residents and they will take anything you throw at them. In the fall, residents can simply rake their leaves to the curb and the city comes by with a big vacuum to pick them all up. I lived close to the city/county line and the streets in the city would always be cleared of snow earlier than the county streets. The city does a lot of the little things right.
The only downside is the number of homes to choose from is less in the city than in the county because of it's smaller size. But if you find a house you like in the city you'll be glad you made that choice.