IPad chooses Wi-Fi or 3G to connect to the Internet depending on its model, a working data plan and an active cellular radio at the time of connection.
Learn more in my article, “Does an iPad Use Wi-Fi or 3G if Both Are Available?“ – published online at eHow.
[...] your Wi-Fi antenna on to use it at every opportunity. Generally, your iDevice will choose Wi-Fi over 3G when both are available. Tap Settings > Wi-Fi > Wi-Fi — tap switch to [...]