With the Arduino serial you need to have an external power supply, and you need to add a voltage regulator to have 3.3V output from the board. I think there are no other differences. I am not sure if MaxSerial has autoreset like the Diecimila. I think it has autoreset, but ask Fundamental Logic about it.
- I'm not sure, but I think that Freeduino Maxserial have a built-in 3.3v. voltage regulator.
- Maxserial uses a max232 to convert ttl to serial, and Arduino Serial uses two transistors.
- Max serial has a pin13Led onboard, and Arduino Serial doesn't.
- Diecimila has TX and RX leds, and Maxserial and Arduino Serial don't.
- Arduino Serial don't have reset and 3.3v pins beside power/gnd pins.