Comparing iPad mini data plans; Why choose AT&T, it has fewer countries and is more expensive?
The value of any particular carrier is based on the performance of the carrier in the area you will use the device.

ie. My area has great AT&amp;T coverage, Good Verizon coverage and horrible Sprint coverage.

You are also only comparing the cheapest tier. Prices change and one carrier may be cheaper than another at a different level of service.