The "problem" is that when you try and view a site on a monitor set to 1920x1080 is that the whole thing just looks too small for most people.
If you've got an image that's 640x480 and are displaying it on a monitor set to 640x480 it will fill the screen. If you try and put the same image on a monitor set to 1920x1080 it fills (approx) 1/9 of the screen.
Going back to your original question-it depends 100% on which monitors you're comparing-size, viewing distance, quality etc.
A 40" LCD "HDTV" IS a computer monitor-albeit one that has some extra circuitry to allow other types of inputs.