HDTV has a built in ATSC HD tuner, enabling it to display OTA transmissions in HD straight out of the box. An HDTV monitor on the other hand has no built in HD tuner, usually they have either a DVI or Firewire port, for attaching a seperate STB decoder. A monitor is of course cheaper.
Your choice depends on your budget and the method in which you will be watching HD. If you will be watching via Sat or cable, then you will need an HD STB in any event. I would say go with the monitor, you can get an OTA receiver later if you require it.