If you have tablets, mobile hotspots and basic phones, chances are you’d want to go with Verizon as the additional fee for them is lower than the standard $40 for smartphones. For each feature phone you’d need to add $30 a month, for hotspots $20 and for tablets only $10. AT&T in contrast has the same fee regardless of the device.
On the other hand, if you really want to use a lot of data, AT&T seems like the only option for the moment as it delivers 15 gigs and 20GB monthly, while Verizon - at least officially - stops at 10GB. Overage fees are set at $15 per GB on both carriers and both carriers include tethering for free.