What is the difference between a fixed lead versus a retractable lead?
The most important distinction is safety! When you choose a fixed lead you buy it at a set length. This allows you, the owner, to set a very clear expectation to your dog. You are the leader of the pack. You need to set clear boundaries so that your dog feels safe and secure.
The average sidewalk is 3-4 ft wide. The preferred length of your leash is about the same. The reason here is that when your dog sees that infamous squirrel or a four-legged neighbor walking by, it is less apt to hop the curb and run out into traffic.
With a retractable leash, you have the ability to lock and unlock the length of the lead.
This not only gives an unclear message to your pet, but also can be very harmful if it sees that squirrel before you do and decides to bolt. The locks on these can malfunction or get stuck just when you need it the most. When you want to reel in your dog, you need to unlock it to get it to retract. If your dog is still excited about getting that squirrel, you will need to pull directly on the cord. Many of these are made with a wired material that can slice right through yours or your pet's skin.
Professional dog walkers are unwilling to use retractable leashes while your animal is in their care.