I should start by saying that I purchased a Wi-Fi only iPad this time around (after buying a 3G iPad and iPad 2) but mostly because I carry a Verizon 4G MiFi hotspot in my backpack and pretty much every where I go. However, if I didn't have a MiFi device, I'd definitely purchase a 4G iPad 3 on Verizon Wireless over the AT&T equivalent.
1. Verizon Wireless' LTE network is blazing fast. I tested it in and around the Philadelphia metropolitan area and I was able to routinely achieve speeds of 15Mbps download and 8Mbps upload on average. Even as far out as King of Prussia, PA Verizon's LTE coverage is reliable and fast.
2. Verizon offers Personal Hotspot service free with any data plan on the iPad 3. AT&T doesn't offer it at all -- even for an additional fee. When enabled your iPad becomes a personal hotspot for connecting up to five devices — such as a MacBook Air, an iPod touch, or another iPad — over Wi-Fi, Bluetooth or USB. Bonus points: the iPad 3 can run for up to 24 hours in personal hotspot mode, which is incredible.
3. Verizon is doubling the data on its $30 monthly plan (for a limited time) netting you 4GB of data for $30 versus only 3GB of data (per month) on AT&T.
Verizon hasn't stated when this deal will come to an end and it isn't reflected on the data plan comparisons on Apple's iPad shopping page:
4. Verizon's LTE network is available in 200 U.S. cities (covering 200 million Americans) versus AT&T's LTE network which is only available in 32 US. cities (covering 74 million Americans). Granted, this a very regional decision and depends largely on where you live and work. You need to check both the Verizon coverage map and the AT&T coverage map and make the best decision that works for you.
I was pleasantly surprised when the LTE icon displayed in the iPad's menu bar last week while I was testing it in southern New Jersey. The area where I live (about 10 miles outside Atlantic City) isn't listed as having Verizon LTE coverage but I was able to to confirm that Verizon engineers are testing LTE towers in Atlantic City.
So there you have it, if you need high-speed data on your iPad 3 Verizon LTE trounces AT&T in pretty much every category.