Just did some research, here is what i found: (mainly about form factor)
http://sherry-aithinker.blogspot.co.at/ ... y-sgs.html
There are altogether 16 pin counts, The external size of the ESP-12S WiFi module is 24mm*16mm*3mm.
http://electrodragon.com/product/esp-12 ... e-esp8266/
* Based on ESP8266 wifi IC
* Fully compatible with forepast versions ESP-12E, or ESP12F
* FCC & CE & RoHS all passed
* Further improved antenna design, better signal performance
Note: They sell the ESP-12S for only $2.40! (This is even cheaper then a lot of 10 at aliexpress right now)
http://tronixlabs.com.au/esp8266/esp826 ... australia/
This has an improved PCB layout and offers better radio performance over the ESP-12F.
Note that the ESP12S doesn't have the SPI pins at the bottom of the board (see the pinout below). You can use these with our SMD adaptor boards.
I might look like a smaller(width) ESP-12 but it's exactly the same as the ESP-12. It has the same physical dimensions and pinout. The breakout pads of the module are now smaller (no more holes, just castellations). This led to a bigger(width) metal housing which probably causes the illusion of a smaller module.
PS: Just saw that they also improved the ESP-07 (ESP-07S).