IPad – Will an AT&T iPad purchased from Best Buy US work in india

If you bought the device without a contract, it's unlocked!

The information about AT&T and Verizon is important when talking about LTE availability in the US. Only AT&T and Verizon provide LTE networks in the US that work with the iPad.

4G LTE is supported only on AT&T and Verizon networks in the U.S. and on Bell, Rogers, and Telus networks in Canada. footnote no.3 - iPad specs

This is important to note as LTE frequency bands used in Europe, Australia and probably most of Asia are not supported by the iPad. You will use DC-HSDPA, HSPA+ or UMTS instead for fast internet access.

Yes, the two models are physically different. The radio is different.

As per Apple's website, the radio frequencies aren't the same. The verizon iPad supports Verizon LTE, and as per this post, AT&T 3G. However, the Verizon iPad does not support all LTE frequencies that the AT&T iPad does.

From Apple.com:

Wi-Fi + 4G for AT&T model: 4G LTE (700, 2100 MHz); UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)

Wi-Fi + 4G for Verizon model: 4G LTE (700 MHz); CDMA EV-DO Rev. A (800, 1900 MHz); UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)

Notice that there are some differences between the listed frequencies. They're nearly the same, but not quite.

Edit:

To confirm, the difference is a hardware difference, not a software imposed limitation on AT&T LTE. From the Macrumors article:

The Verizon iPad hardware supports only the 700 MHz band for LTE to match the carrier's technology, while the GSM version sold for use on AT&T's network supports both the 700 MHz and 2100 MHz bands as used by that carrier.

If you're looking to travel internationally, go with the AT&T iPad. (I got the Verizon iPad because I wanted the free hotspot feature, which AT&T does not offer for free at this time.)