There will always be a debate on which exercise is best for a certain muscle. It is imperative however that people understand that a muscle is composed of more than meets the eye. Many times, a muscle if actually a group of muscles and is composed of sections and areas.
This is where the debate of which exercise is best takes on a whole new perspective.
When looking at a muscle like the traps, understanding the lay out of the muscle will better inform you on how to effectively train it (as with any muscle that is).
The traps are a diamond shaped muscle that are made up an upper, middle, and lower section of the back. When training the traps, the shrug reigns supreme. The barbell shrug however has two different variations- the front shrug and back shrug.
The front barbell shrug is maybe the most popular shrug movement. With feet about shoulder width apart and knees softly bent, grip that barbell right outside of shoulder width and lift the shoulders up to the ear, squeezing at the top. Slowly bring the weight all the way back down and repeat the rep. Since the weight is in front of you, more control is available which in return allows you to use more weight. The emphasis of the front shrug is on the upper traps which are the most visible from the front of the body.
The back shrug has gotten more and more popular over the past few decades. Standing in front of the barbell (having it sit right below the waist down around the butt), grip the bar with an overhand grip shoulder width apart. Keeping the knees softly bent, focus on bringing the shoulders up to the ear. Doing this movement on a Smith Machine allows you to lean forward more and get better control and range of motion. Since the weight is behind you it is not as easy to lift a huge amount, but the Smith Machine can allow you to heave more weight safely. This movement hits the upper traps as well, but not as much as the front shrug does. The back shrug puts more emphasis on the middle part of the traps.
Is One Better?
It is hard to say if one movement is better than the other. It truly depends on what you are going for. If you are looking for monster traps that everyone can see when they look at you, then front shrugs are for you. If you are looking for more thickness in the middle part of your back, do back shrugs.
Another suggestion is, do both! There is no rule that says you have to pick one or the other.