Generally speaking, the decision should be based on the location served--so if the company does business in the entire Dallas area then I would use Dallas. But, if it's only in Farmer's Branch, then typically you'd choose just that city.
Another decision would be whether or not this business is going to have (or currently has) more than one location in the area. So, if in the future they will have a Dallas location, a Farmer's Branch location, a Plano location, etc. then that could cause an issue in the future, as you'd need to fix it.
If the Farmer's Branch location serves the entire Dallas area, then you should probably go with Dallas.