There isn’t a binary answer - as your needs will dictate which solution is the best fit for your needs.
An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services.
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.
More information about Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
WiFi Calling
Make a call with Wi-Fi Calling – Apple Support
The most flexible solution is to have both an iPhone and a WiFi+Cellular iPad - as this allows the iPad (either with or without a SIM) to access cellular calling and messaging via a associated iPhone, along with cellular data. The iPad Cellular data can either be tethered via iPhone, or use the iPad cellular data if provisioned with a DataSIM.
Many Users choose to have a WiFi+Cellular iPad to benefit from GNSS positioning - tethering to their iPhone for Cellular data and access to Cellular calls and messaging. For many, who already have an iPhone, this provides a cost effective solution offering the best of both worlds.