I made a spreadsheet of iPad versions for a CAP friend who found out, the hard way, that he'd bought the wrong iPad for flying purposes. He intended to buy an iPad with 3G capability, because [whether or not you activate 3G service], 3G models have a GPS receiver. But a BestBuy sales droid convinced him to buy a non-3G model and add a mifi device that connects to Verizon. While that can help you create a small (4-5 machine) slow-speed WiFi network in a place on the ground that doesn't have any WiFi, it does nothing for you in an airplane because most of the time you won't get cell tower signals. In an airplane, you need to receive signals from GPS satellites.
The issue of whether you can benefit from an additional GPS receiver with WAAS capability, either plugged into your iPad, or connecting through the air [short distance] via Bluetooth, is a separate issue. One friend has a Bluetooth GPS, I bought the plugin kind, and I may switch to the Bluetooth variety. The GPS inside my iPad works fine, but the extra receiver gives you better reliability and precision. The plugin kind gets power from your iPad but itself can be plugged in to your cockpit's cigarette lighter keeping your iPad charged; its main drawback is that you can't use it with a certain kind of knee strap. The Bluetooth kind needs to be charged before you fly.