From my experience, to get the best performance on WiFi you need direct visibility of the point you're trying to reach. The signal weakens very fast and can't get very far if there are walls, buildings or other obstacles between points.
3G/4G/HDSPA/WHATEVER can penetrate easier and use less towers to transmit.
Disclosure: I have no idea of what I'm talking about.
Does anyone know how using empty/unused TV spectrum plays into this? Seems like you would have far better success since they would be able to penetrate walls and other obstacles better than regular Wi-Fi signals can.
I know there was some research being done at Microsoft at using this as sort of a long-range wi-fi network but short of an installation at their corporate campus, I haven't heard any news about it lately.
3G/4G/HDSPA/WHATEVER can penetrate easier and use less towers to transmit.
Disclosure: I have no idea of what I'm talking about.