There is no difference in value between a town building and a city building. A building is a building. The key is to get as many as possible with the fewest bombs possible.
Obviously, bigger bombs take more buildings at a time than smaller bombs, although sometimes, with the right concentration of buildings, dropping 2 or more bombs is actually beneficial as the proximity damage area between the bombs seems to be intensified, dropping more buildings with, say, two bombs spaced than you could with one, then another in another area.
Buildings and hangers work differently. If you drop 1 1000 lb bomb and take out 4 buildings you get a 400% damage ratio. If you drop 3 1000 lb bombs and take out a hanger, it's closer to 100% due to the hardness of the hanger.
So, in conclusion, to maximize your bomber score, hit targets with high concentrations of soft targets with as big of bombs as you can carry and space them properly for maximum proximity damage.
[EDIT] BTW, points work differently between buildings and hangers also. As an example, I used to fly Lancs against VB's a lot. I could take out 9-12 hangers (on the old VB layout) with 14x1000 lb bombs x3 for ~400,000 points (we'll average a bit and say ~38000 points per hanger or 2040 points per bomb). Now I usually fly AR234's against strats taking ~21 buildings at 250,000 points with 3x500 Kg (slightly greater than 1000 lbs) bombs x3 (~12,000 points per building or 2800 points per bomb).
These are just estimates but as you can see, the points per 1000 lbs of bombs remain relatively stable (within range due to estimations) while the points per object do not, so, you need to kill several (3-4) buildings to equal one hanger from a points perspective but hitting the buildings adds the benefit of a higher hit %.