Career extension trainer is the one you get when your (key) players are 33+ up and you dont want their skills deteriorating fast. Basic is fine if you are not training anybody. No difference between a world class and basic in terms of skill drops with career extension just price.
On the other hand, if you are training a rookie prospect go with what you can afford. 4 is obviously easier week to week to maintain, 5 is slightly faster in terms of skill development. Are you training the player so he can become a key guy in your rotation or is this for NT consideration? Your goal and finances will answer your question.