mini ipad; wifi only or wifi and cellular?

Depends entirely on where you want to use, the availability of WiFi for you, and what you intend to do on it. If you want the "freedom" of cellular, and you are willing to pay for it, then obviously that would be the path you should follow. Other than that, there really isn't all that much difference between the two. I've managed with WiFi only on both of the iPads I've owned just fine, but that's me.

In case you're often out of reach of WIFI networks and need an internet connection it might be an option to add a cellualr modem as well.

Some solve internet connectivity by setting a cellular phone with data plan in thetering mode and connect a tablet to the internet over the cellphone.

There is one catch in this situation.

When the data plan has limited monthly bandwidth, its a bit of a struggle to control the bandwidth consumpion of the ipad. Since it sees a WIFI connection the iPad will use it in an unlimited way over your cellular phone. Such as cloud backups and much more stuff going on in the background.

When the ipad has a cellular modem, there are controls to turn off cellular internet access for each individual app and thus control the cellular bandwith consumption.

I travel a lot and use prepaid data simcards with limited bandwidth and therefor decided for an iPad mini with cellular modem.

Here's more about the iPad and another detail about SMS text messaging;

iPad Mini with cellular modem