This is not scientific, but it is something I've experienced. More than once I've been in a location that had both 3G coverage (4-5 bars) and a wifi connection and yet it seems the response in downloading data was slow. I would turn off the wifi, still getting good 3G, and it performed better. Is there some sort of conflict that results with access to both networks? It is possible that it was just a fluke in those instances, but I do wonder. At home, where the phone network is not as good, I don't think I've noticed it. In this case I would think it is using the best, which is always the wifi.