The main difference between crew and association is association is a much more robust feature and experience than crew and is more the industry standard (franchise type modes)
I really hope they get crew back in 2k14 for you guys better than ever but taking out crew is kind of a sign from 2k that fixing it is going to take a lot of time effort and smart thinking where as Association has been sitting here close to perfection for a min now watching its sister mode in myplayer get some cool new features while it still struggles with broken AI.