I don’t have a smartphone, but I do have a 3G Kindle, which switches automatically between wifi and 3G data like phones do. Unlike a phone, however, I can’t turn the 3G and wifi on and off independently; either wireless is ON and it uses whatever carrier it can find, or it’s OFF and connects to nothing. (Less annoying than it sounds; there are no per-megabyte data charges for 3G browsing on Kindle.)
I can walk to the end of the house that doesn’t have the wireless router in it, and whether I end up with one bar of wifi or all five bars of 3G, or something random in between, depends on time of day, temperature, humidity, RF interference, phase of the moon, price of tea in China, and about a thousand other things I don’t have enough data to figure out.
In other words, it depends on what gizmo you’re using, how you’re holding it, where you’re standing, and what mood the cosmos is in lately.