From the new crew 2.0 video:
Normal Instructors give 1 small skill improvement and 10% exp bonus. They only take one of the 4 instructor slots for the nation so you get 40% exp bonus and 4 small skill improvements.
Special Instructors take 2 slots and give you 40% exp and 2 big bonuses to skills so you get 80% bonus and 4 big skills.
Unique Instructors take all 4 slots but give "great" bonus to 3 skills (looks bigger bonus than the special Instructors) and 100% bonus exp.
So next time they will be sold just fill your barracks with x10 times of the unique crew in the offer and you prepared for 2.0 I guess.
Edit: One little thing I saw on the 2nd time watching is that unique crews can't choose the skill they boost but are given a meta skill for the tank class you chosse.
This could mean sometimes it could be better to take special Instructors and give them the skill you want them to have.