Well of course the Roboduino and Axon are just smaller or bigger brothers - respectively.
Depends on how many I/O lines, program memory, RAM you need. The Axon has the most of all.
If you want to program in C++ or C then either is fine. You will find links throughout this site and tutorials for libraries for both.
Any code examples can easily be converted to any AVR board from the $50 robot board onwards.
Depends on your hardware budget