We always use two fins in the course. That’s because it’s more convenient and two fins are more intuitive for diving. Some training is required before diving with a mono fin—unless you can tap into your inner dolphin.
You’re simply more mobile in the water with two fins. Two fins (also called bi- or stereo-fins) is great when it comes to agility: during course, the training, as a safety diver for your diving buddy, covering long distances and underwater shoots.
One fin (also called a mono fin) is great when it comes to efficiency in a certain time. For instance you are doing an extreme deep dive or want to dive a very long distance underwater. The diving movement is similar to the way a dolphin moves and it streamlines your dive. But using a mono fin takes practice and you loose maneuverability because both feet constrained. That being said, if you can handle the drawbacks, a monofin can also be great fun for a true freediving experience and discovering the flora and fauna. The elegant motion and effortless glide offers a unique experience in the water. In the end, it’s not what’s better, it’s about what kind of experience you after.