Out of the blue buy iPad wifi vs. 3G, in case you decide to go for the wifi+3G version (depending upon your business requirements), you must make sure to purchase a model which corresponds with the carrier that you would prefer to use for your 3G services. AT7T and Verizon are the major carriers available and every wifi+3G model of an iPad is specially configured so that it can be used with either of these two carriers, and not both of them. Hence, depending upon the carrier you prefer, the right kind of model must be selected, even if you do not activate the 3G services right after your purchase.
