The Verizon and AT&T iPad 3's with 4g are exactly the same. I bought an iPad 3 in Miami last week. The apple store only had the Verizon model in stock. The manager assured me that there was no difference in electronics or antenae...that the only difference was a mini chip from Verizon vs a mini chip from AT&T.
I was a little concerned as I live in Brazil and planned to use an Oi Velox 3G chip which operates on the AT&T frequencies rather than the Verizon frequencies. Thankfully the iPad 3 is working perfectly. This proves that the Apple store manager was correct and that there is no difference between the Verizon and A&T ipad 3 versions. It also means there are a lot of folks putting incorrect information on this topic out on the Internet.
Mark