Which leash you should choose depends on many factors. Here, the area of use and especially your personal preferences are important.
Shorter leashes are very suitable for the city stroll, public transport or even crossing streets, etc.. Here the focus is on safety and direct impact. Shorter leashes are also particularly suitable for training with your dog and obedience.
Adjustable leashes are particularly well suited for daily walks. They can be adjusted depending on the situation, but are rather not suitable for traditional dog sports.
Longer leashes are useful, for example, when a leash is not possible, but you want to give your dog more room to play. They are also particularly suitable for training the hunting instinct, tracking work or mantrailing.