*) v6.x STM32 as the main controller, which has a built in CAN controller
*) v6.x STM32 talks to RN52 at 115200bps; v5.x ATMEGA328 is configured at 9600bps
*) v6.x STM32 has a factory built-in bootloader that listens to UART1; v5.x ATMEGA328 has no bootloader by default
*) v6.x STM32 uses RTOS for dealing with time-critical event handling; v5.x uses custom functions for that
*) v6.x firmware has some added features for handling SID text control (automatic character mapping, timing, prioritization, etc.), that v5.x does not have
*) v6.x firmware code is portable and is being used in BlueSaab XM project as well
These are a few I could come up right from top of my head.
2001 9-5 SE V6; 2006 9-5 Wagon; iOS; BlueSaab version = "latest and greatest" :)