I know the A1387 model is sold elsewhere, and the A1431 in China, and they are both 4s, but I would like to know any technical differences between the two, i.e. what makes the China model different than the other ones?
China around the iPhone 4s and 5 used WAPI variant of wifi and a country specific CDMA network. The A1431 is tailored to those differences.
The China-only iPhone 4S still is believed to be a "world phone," supporting both GSM and CDMA networks (UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz), and CDMA EV-DO Rev. A (800, 1900 MHz), and the "antenna automatically switches between send and receive."