Many of you have asked for a side-by-side comparison of ArduPilot Mega and basic ArduPilot, so here it is. Note that with Mega, we moved the servo and GPS connectors to horizontal rather than vertical placement, to make them more robust in the case of a crash.
As you can see, the main processor in ArduPilot, an Atmega328, is just handling RC input and the failsafe function on Mega, while the main CPU has been upgraded to an Atmega1280, with more than twice as many I/O pins. In a sense, every chip has been upgraded to their big brother on Mega to handle more channels, sensors and communications paths. Basic ArduPilot, especially with the Turn-Rate Limiter Shield
, will continue to be sold and will be the best plug-and-play solution for newcomers, while Mega will offer more capability for more advanced users.