A lot of computers don't provide enough power to their USB ports to charge the iPad at a sufficient rate, so the wall charger is usually the most efficient way to charge the iPad. From http://www.apple.com/batteries/ipad.html :
The quickest way to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter. When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode. Make sure your computer is on while charging iPad via USB.