For those who are not legacy users, the lowest users and the extreme users are going to prefer Verizon pricewise. The bulk of users will prefer the 3 Gb from AT&T as a better buy than Verizon who allows only 2 Gb for the same $30/mo. Coverage is anyone's guess, because it is constantly changing. Verizon has a head start, but AT&T seems to be rolling out LTE faster and may catch up. Where 4G LTE is not available, AT&T has faster 3G and 3G based HSPA+. Look at the coverage for both in your area of use, what they have announced as coming, and the reputation of each in your area.
In my area, I am using AT&T for my iPad and the 4G service is fabulous from the short time I have used it. I also really enjoyed the 3G service on my old iPad. Two years ago, it was new to the area, and within a year, it was everwhere I travel, which is a lot. But, I also use Verizon on my cell phone and and I have no issues or complaints with their service. It is not a smart phone, so I cannot compare data coverage. However, I travel frequently into Colorado. There are large areas with one or the other coverage but not both. I often wind up swapping between my iPad and cell phone when traveling there. Makes carrier choice a local issue more than anything else.