what is the difference between A1459 and the A1460 models of iPad (4th generation) Wi-Fi + Cellular?

Different model

http://support.apple.com/kb/HT5452

They support slightly different cellular bands - from http://support.apple.com/kb/SP662 :

Model A1459*

• GSM/EDGE (850, 900, 1800, 1900 MHz)
• UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz)
• LTE (Bands 4 and 17)

Model A1460*

• CDMA EV-DO Rev. A and Rev. B (800, 1900, 2100 MHz)
• GSM/EDGE (850, 900, 1800, 1900 MHz)
• UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz)
• LTE (Bands 1, 3, 5, 13, 25)

From that it looks like A1459 is AT&T and A1460 is the Verizon and Sprint model : http://www.apple.com/ipad/LTE/