I think it depends on the car and wear the rust is. In the case of mine for example it's a combination of both. The lower sections of the floors like the area at the driver's and passenger's feet and the area under the rear seats is bad from the windows and t-tops leaking while the rockers and braces (and the floor in those areas) are rotted from the underside from either things like road salt or from the time it sat in a field. A lot of time if you pull the carpet up and compare the visible rust on top to that underneath you can get an idea if it came from the inside or out.
