A good DJ will provide you with a large list of many types of music to select from. He will ensure that the music you choose will be played. A list will also reveal your DJs library size, make up and capabilities. The larger the variety, the greater your selection. The more shows your DJ has performed like yours, will be a good indicator of his experience.
More experience ensures a better show and a little higher cost.
Saving a few dollars on an important show may result in something less than your expected. And it could ruin your events. Ask for references. Your DJ should take an active part in helping you plan the format and timing of your event. He will work with you to organize all activities at your event and then remind you when each is to take place.