re: serial v/s usb.
If you have a serial port, you can save yourself $7 and get the serial version. I cannot guarantee it will work with the palm yet, but certainly it will work with the standalone MPGUino, and why make end users pay for a usb port that they will only use once to program it?
Of course if yoru pc/laptops are legacy free, you might want to get the usb version.
If you want to prepare for all support contingencies, then get one of each and possibly the rs232 card