Other than adding flexibility of Cellular capabilities, should they be required either now or at a later date - a significant feature, often missed when making comparison between WiFi Only and WiFi & Cellular models of iPad, is GPS Positioning capabilities.
Only WiFi & Cellular models of iPad include GPS functionality.
In more detail...
The WiFi & Cellular models of iPad include GPS positioning capabilities - so the iPad can be used, for example, with [moving] Maps and other location-based services. The iPad can accurately determine its location, using GPS alone, independently of any WiFi or Cellular network connection. Accurate determination of location also affects automatic determination of time and timezone
By contrast, WiFi Only models of iPad can only estimate location based upon lookup of the public IP address of the WiFi network with which it is being used - or though lookup of a database of surveyed WiFi neighbouring WiFi networks. Derivation of location using this technique can be reasonably accurate in urban and metropolitan environments - but is much less accurate (if it works at all) in rural and less populated areas. Without an active WiFi connection, the iPad cannot estimate its location.
Choosing the slightly more expensive WiFi & Cellular model provides flexibility in choice of network connection - and accurate determination of location - wherever you are - or whether you have any network connection. Just because you choose the Cellular-capable model does not commit you to using Cellular connections.
My personal advice, so as to preserve flexibility, would be to choose WiFi & Cellular models of iPad in preference to the WiFi Only. I am yet to see a complaint from anyone having the Cellular feature where it isn’t needed - but regularly see reports of disappointment and frustration at learning, after the fact, that the iPad lacks accurate GPS positioning capabilities.
I hope this guidance and clarification is helpful in providing a better understanding of some key features of iPad.