iPad pro

Here are the major differences...

The WiFi & Cellular model provided the flexibility, should you decide to use it, to use Cellular connectivity in addition to WiFi for connecting to the internet. The WiFi Only model is limited to just using WiFi networks.

The WiFi & Cellular model also includes GPS positioning capabilities - so the iPad can be used 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 its 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 information helps to constructively inform your choice of iPad should you be considering purchase - or in clarifying functionality should you have already purchased an iPad.

The answer to your follow-up question is not a simple binary yes or no response. Read on...

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone.

However, If paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network - but this is not the end of the story.

Now, if your both your Cellular Carrier and your calling plan support WiFi Calling, your iPad can make and receive calls independently of the iPhone - even when the iPhone is switched off, elsewhere, or using a different WiFi network. Not all carriers support full WiFi calling - and many Carrier representatives don’t technically understand or acknowledge its existence.

[NB: Many contributors here don’t understand the full implementation of WiFi Calling either - perhaps because this feature is often only available from some full-service cellular carriers; cut-price MVNO cellular operators may not offer the service at all].

More information about Continuity and the relevant Cellular services can be found here:

Continuity

https://support.apple.com/en-gb/HT204681

SMS/MMS Messaging

https://support.apple.com/en-us/HT208386

WiFi Calling

https://support.apple.com/en-gb/HT203032

When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.

I hope this information and clarification is also helpful in informing your eventual choice of iPad.