What's The Difference Between iPad Wi-Fi and iPad Wi-Fi + 4G?
The Wi-Fi model is only capable of connecting to the Internet where a wireless hotspot is available (like your home, a coffee shop, airport, etc.).
The Wi-Fi + 4G model is able to connect to the Internet through a wireless hotspot (Wi-Fi) but also supports 4G wireless access from a cellular provider such as AT&T or Verizon. 4G service allows you to stay online (almost) anywhere. If you choose to buy a Wi-Fi + 4G model, there is a higher upfront cost, plus AT&T/Verizon charge a monthly fee for the service.
Unless you need Internet access everywhere, the Wi-Fi model is the best bet (and the most popular option).