Show versatility, professionalism and warmth. They will remember what a pleasant experience it was to work with you. Neither one will guarantee that you will get work, but both have merit. A performers demo tape is usually the first way a producer will hear what your voice sounds like. Beginners don't have the samples and many pros find that over the years they become type cast in a style and must create new material to keep their tapes fresh.
Some people have several different demos for different markets.
If you have a good range and can do commercials, animation, promos and narrations, you may need four separate demos. Your tape may be your only contact with a potential employer. While practices vary from city to city, most casting, at least initially, is done from demos. Your's has to sound just as good. Your demo must be of such a quality that the producer will want to listen to the whole thing.