The rule of thumb I recall was BSII until Jungle maps arrive, then the 73 (or was it 63?) charisma map instead. If you are looking for specific class fodder, the other desert missions may be better.
Each fodder level is 5 xp, so going from ~40 charisma to ~60 charisma for ~50 extra xp per fodder is not worth it.

Hopefully someone will have a proper link about it, since that is what I recall reading, and my memory might be faulty.