This is an important distinction. A smart author or band will choose a preview that really shows off what the song is going to do. Combined with the first minute preview, that should be sufficient to tell most people all they want about the song. If I hear a preview that sounds exactly like the demo music, then I sometimes assume that the song doesn't really "go anywhere" from the demo.
That is EXACTLY why we chose the preview we did for "The Heist". We knew people would see [and LIKE?] the intro and part of the first cycle so highlighting the end of the drum solo and beginning of the guitar solo would show people that there was plenty of fun on the way.
Morituri Nolumnus Mori - We who are about to die don't want to.