Wifi uses less power than 3g so you want to leave it on. My battery dies faster if I shut it off.
I have no WiFi at home and any place that has "free" WiFi prevents me from doing anything more than checking email. With no WiFi available it'll be using 3G anyway even if I leave it on. As for power usage, a lot of that depends on signal strength. A good 3G signal will use less power than a poor WiFi signal and vice versa. Of course, with WiFi you should be finished quicker so the power savings using 3G instead would be moot.
When I have access to Wifi that requires a login, yeah, I'm all for it. To be honest, I probably wouldn't notice any extra battery drain if I left it on, but since I have no place to connect it to, I don't bother.
To keep this on topic I've deleted Advance Task Manager, although I do have System Panel if I have to kill any apps.
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