Hi Lanzfeld,
Rookie vs Veteran sets the experience/skill level of your new pilot.
As an example, you may find as a Rookie, that you are not given the latest aircraft type to fly. Once you prove yourself, you may be given newer aircraft and/or promoted etc. It does not affect how you actually fly/control your aircraft - that's still down to your personal skills.
Enhanced AI (Artificial Intelligence)- this enhances the experience/skill level of the other 'game controlled' aircrew and makes your missions outcome more unpredictable.
The FAQ under the Help link on the OBD site gives a lot of information:
http://www.overflandersfields.com