I have a chipkit max32 and tested it with some sketches, and if there are no AVR low-level code in it, it works the same as an original Arduino. The chipkit has more IO ports, and CAN and ethernet. MIPS is a nice architecture, the Sony PSP uses this core, too. But from C you don't see any difference and if you don't need the extra pins and interfaces, maybe the Arduino is better, because there might be more low-level libraries released for it (tricks like FBAS signal generators etc.).