Decide upon the length of the reception. Bands tire more easily than DJs and may not be able to work as long. Consider the location of your reception. Factor in the level of formality. Live music is often considered the more traditional and dignified choice.
DJs are the rowdier, hipper option.
Listen to them play. Sit in on an event, attend the band's auditions or ask for a recorded tape. Check out the bandleader. One benefit of a live band is that the leader can double as a master of ceremonies. Question the DJs about their album collections.