As long as wi-fi is available, the iPad will use the wi-fi.
As for the iPhone, it also uses wi-fi whenever it is available, not you cellular data (with the exception of cellular phone and SMS services, which are always on). The new LTE voice call services that will be available from some carriers, soon, will probably change that; but I'm not sure in what way, or how it will impact data limits.
In both cases, I believe the devices conserve power wile the screen is off (and the device is not plugged into power). Probably by a combination of limiting transmission to the minimum needed to keep the connection live, and only checking in with the network every so often.
Anyway, I'm pretty sure that's how it works.