Actually I don't think it really makes that much of a difference. The Traps are a part of the shoulder girdle and also the upper-mid back area, a pivot point for shoulder or back work. The Traps diamond shape is quite functional in either case, it's like holding or linking those two muscle groups together.
You are never going to isolate any muscle group from another, anyhow. Some work traps with shoulders, others with back...your choice. BB'ers tend to over think (or under think) fairly simple exercises protocols If one or the other works best for you, than keep doing it that way. Or until something better might come along.
Shrugs are the purest form of a direct trap exercise. The old Hise shrug, perhaps #1 in that regard. Guy's also used the old style calf machine, with the yoke placed on the shoulders, and did shrugging that way...another version of the Hise shrug. Hi-pulls, with a wider grip, an excellent trap movement, as well as hitting the lateral and rear delts, lats/back and biceps. Cleans ranks a close second (my view only).
One of the better mass building compound exercises to hit the traps and back/shoulder area is the BB row & Hi-pull (heard it called different names in different gyms). Starting with a regular BB row, the bar is raised to about knee height. At that point the upper body begins to slowly raise into a standing position, while the bar is still traveling upwards. At the near standing position, the bar is being pulled waist high. At a full standing position, continue the pull until the bar is somewhere between the naval and lower rib.
Lower the bar and repeat for 6-9 reps of 2 or 3 sets. Always keep the bar close to the body when doing this exercise. Almost touching the body actually. Can work up to some very impressive weight with this exercise, the byproduct being more muscle mass.
Good Luck.