The main difference is that a Sportive has a marked route and is marshalled, there are mostly mechanics on duty at several points should you get machine problems. In an Audax you get a route sheet sent to you before the start but you are then responsible for finding your own way round, there are no route markings. You are also expected to be self sufficient and be capable of sorting any bike problems yourself, there is no backup.
Audax also attracts a percentage of weirdos who foam at the mouth and shout at anyone whose bike is newer than 1955 and is not dressed tweeds. Sportives attract a percentage of people who think they are top class racers but who have never had the courage to pin a number on their backs and do it properly.