Some folks over at Phonearena created a handy comparison chart showcasing the differences between the Verizon and AT&T shared data plans (shown in the chart above). For the most part, they seem fairly even, although in some cases (depending on the number of devices) one or the other is cheaper. The main difference comes with the type of devices that you use on your plan, since AT&T charges the same for tablets as they do for their phones, whereas Verizon only charges an additional $10 bucks per month for each tablet. Here's a quote with some more info,

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.
Source: Phonearena