Head halters are still a useful and humane tool for teaching loose leash walking. As with any of the tools we use (including leash and collar), they are a means to an end and should not be considered as a replacement for proper training. You still have to do the work!
There is no "magic" product out there to solve your problems. They are most often considered for dogs who have developed a pulling or lunging problem that makes them difficult or unsafe to walk (as in the case of the 100# owner and the 80# dog). They allow the owner to turn the head and redirect the dog's attention. They are extremely useful in dealing with dogs with aggression issues. They are not a muzzle, although many people will assume so if your dog is wearing one... consider it an opportunity to educate the public :-)
The two most commonly seen and available head halters in the USA are the "Gentle Leader" (GL) and the "Halti". They each have their advantages and disadvantages. The general premise is that they work like a horse's halter. If you can control the head, you can control the body. Some dogs are very resistant to them (often the ones that would benefit the most) and some accept them with little adjustment period. You can increase your chances of success by introducing the halter in a slow and positive manner.
Gentle Leader: Manufactured by Premier Pet Products, they were formerly only available from trainers and vets, but are now available in many retail places. They come in various sizes and colors, but I suggest choosing a color that closely matches your dog's face.
Description: Two "loops" that connect below the chin at the throat. The "neck loop" fits like a collar high on the neck just behind the ears. The fit is quite snug. The "nose loop" goes over the dog's nose and rests just in front of the eyes. It is adjusted fairly snugly so that the loop won't slide off the end of the dog's nose. There is a ring at the bottom where the two loops connect. This is where the leash attaches.
Advantages: Well made. Nice feeling fabric. Choice of quick release or buckle. A second ring on the neck loop allows you to use it as a collar (nose loop hangs loose at throat rather than over nose) in transitioning off of using the GL. For dogs that accept it readily, the owner can be walking the dog in a matter of minutes. It doesn't require much training for owners to get started.
Disadvantages: Snug fit is fairly restrictive. Some dogs find this objectionable. Generally costs more than the Halti. The position of the leash attachment gives less control than that of the Halti.
Halti: Created by Dr. Roger Mugford and manufactured by Coastal. They are available in many pet supply stores and catalogs. They come in a variety of sizes, originally only in black, but some colors are now available. If you choose a color, choose one that blends with your dog's face.
Description: One piece design has cheek straps that connect the nose loop and neck loop portions. Newer models may have some adjustment options. Leash connects to a ring that is located on the bottom under the dog's chin. All have quick release connectors. Fit should allow a finger to easily slide under both cheek straps.
Advantages: Looser fit and lighter weight fabric is less restrictive and better accepted by some dogs. Location of the chin ring gives better leverage... easier to turn the dog's head without twisting the neck. Less expensive and may be easier to find than the GL.
Disadvantages: Feels a bit flimsier to the touch. Looser fit means that the dog can paw it off or back out of it easier. It is recommended that a back-up leash or collar be used for safety and the Halti now comes with a short safety strap to attach it to the collar. Lack of adjustment means it may not fit all head shapes.
My Opinion: In most cases, I prefer to use a Halti over a Gentle Leader. I like the more relaxed fit as it is accepted by more dogs and the placement of the leash attachment gives me better control. I always use a double-ended leash with head halters to prevent putting steady pressure on the dog's head, which is the main reason dog's object to head halters. This double-ended leash system is also safer and gives me much better control with less effort.
SAFETY NOTE: A head halter should only be used with a short leash (6 ft. max). Use with a longer lead could allow the dog to run to the end and be jerked by the head. While rare, this could result in a neck injury. The head halter is a training tool and should be used properly at all times.