With T-Mobile, I have unlimited data on my iPhone and iPad. But limited tethering data. My iPad data plan is $20-$25 a month.

Except the iPad battery will last longer and will not kill your phone while you do stuff

>inb4 buy a battery bank

Hmm, I will be honest, I was either always close to a power outlet or it lasted long enough for me. You only need to tether if there is no Wi-Fi.

I have a 20000mAh with me when I intend to work remotely though. Either way cheaper than paying for another plan for iPad and buying a more expensive iPad.