There are alternatives available to let you stick with the cheaper Wi-Fi model of the iPad, yet still connect while on the go. If you happen to have a jailbroken iPhone, you can use the MyWi app to allow the iPad to connect to the iPhone via Wi-Fi and ride on its 3G connection.
You can accomplish the same goal with a 3G-enabled Windows 7 notebook. An undocumented feature of Windows 7 allows it to share out its Wi-Fi network and become a mobile hotspot. Or, you can invest in a device like the Novatel MiFi--with connection plans available from Sprint or Verizon.