Seeing as how pranayama is mostly controlled breathing, and Zhang Zhang (standing like a post) seems more like static muscle resistence or isometric weight training,
It actually has quite a lot do with Qi when taught properly - good ZZ, as I understand it and have been taught/practiced it, is all about optimizing your posture to align your body for optimal flow of energy. This allows blockages to clear up, improves health and overall energy levels, releases physical, emotional and psychological tension, improves posture, etc. Just standing in ZZ for 10 minutes or so will induce quite a strong sensation of energy moving around the body for me.
PRBH - I think it comes down less to the difference between Pranayama and ZZ/Qigong and more to the difference between individual systems of practice. A good system of pranayama will be better than a bad system of Qigong, and vice versa - taking into account what each specific system achieves and whether that is in line with your goals.
The main difference, broadly speaking, is that ZZ/Qigong are inextricably tied into the culture of martial arts, even those systems which aren't inherently martial are informed by Chinese martial arts culture and training (which can be both good and bad), whereas pranayama is more purely meditative/yogic. And I haven't done a lot of pranayama, but I understand that it is generally practiced while seated and still, while ZZ/Qigong is mostly standing and, for Qigong, frequently moving.
