The important thing about a demo is to make yours as near to the finished article as possible. If the finished song will be aimed at a simple guitar voc performance then the demo should be a quality produced and performed guitar voc. If the song is a full production with all the bells and whistles then make sure the demo contains all the bells and whistles. Marc is quite correct.... gone are the days when people heard beyond a simple amateur rendition of a song. With so much competition producing radio ready demos you have to compete on an even playing field by making your demo as good and professional as possible like the many other thousands submitted by the competition. It is worth noting though that few big hits nowadays are just simple guitar vocs. Most big hits are production driven rather than relying on just melody lyrics and a simple performance.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055