cmd is NT's actual command processor, while command is a 16 bit app, sort of a "virtual DOS", MS even has a fancy name for that, something like NT Virtual Dos or something.
Command will pass command on to cmd for processing, so there's no advatnage to command really, unless you need to run some 16 bit app or something.
Command is much slower though, as you'd expect from an emulated app, so for general purpose use, cmd is the one to use.