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.
MaxSerial has the 3.3V option ($ 1.15 additional) and has auto-reset (according to information in Fundamental Logic's web site).
Thank you for your reply.