Flexible ducting is usually used in residential applications and is available in a variety of materials. The most common type is the fiberglass flexible duct. It is lightweight, easy to install, and resistant to moisture and fire. The other type of ducting you mentioned, "complicated duct", is typically used in commercial or industrial settings. This type of ducting is more rugged and can handle heavier traffic. It's also better at keeping contaminants out of the airstream.

There is no definitive answer, as it depends on the specific application and needs of each individual project. In general, though, flex duct is often preferred due to its flexibility and ease of use, while complicated duct can be more difficult to install and can be less forgiving if there are any mistakes made during installation. Ultimately, it's important to consult with a professional HVAC contractor to get their expert opinion on what would work best for your specific situation.