I have both - I really like Annick Goutal fragrances in general, but these two are very interesting.
Les Nuits resembles Eau d'Hadrien, but is a little bit more ambery and deeper, and lasting longer on the skin. It's like a mysterious counter-part to Eau d'Hadrien. It's practically become my signature fragrance now.
Eau du Sud is a slightly stranger beast... It has the same citrus opening of the above AG fragrances, but it fades rather quickly and there is a muted green coeur. It dries down to a nice woody scent, but it is very, very faint - you'll need to put on quite a bit of the Eau du Sud for it to be noticed.