Arduino software uses a higher level C programming to make it easy for beginners. Axon uses a lower level C that all professionals use to program their microcontrollers.
Axon uses the same microcontroller as Arduino Mega uses, just has a bit less flash. They both can be programmed in Arduino, but Axon needs a few mods to the files before you can use it. I plan to make the changes in the next few weeks. The difference between them is Axon uses a different USB chip so uploading programs to the Axon from Arduino software will not work directly. You will have to use an external uploader (the one Admin provides) after you compile the hex file with Arduino software. If I find an easier way, I'll let you know.
For robotics, Axon has the 3 pins headers but Arduino Mega does not - so you will need a shield and the available shields were not designed with this purpose in mind.
To resume, both of them have advantages and dissadvantages. If you dont mind making a shield with 3 pin headers, go for Arduino Mega. If you don't mind having to upload the hex file with an external uploader, go with the Axon. Besides, with the Axon you can allways step up to the real C programming and use Admin's Biped Engine for servo control and many other examples on this site. I guess with the Axon you can have the best of both programming environments.