GTForce

The clean is an excellent exercise for the traps.

If you performed it with a press then you would be getting the best of both. The clean & jerk (or press) is a phenomenal exercise, the only requirement is the patience to learn, coordination to perform , and you must have flexibilty in your wrists.

You can perform it as a full-body exercise (from the floor, w/the overhead press) or, specifically for traps, you can do standing power cleans.

An additional benefit is that the rotator-cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor) will be strengthened as opposed to potentially damaged with the behind the neck press (which primarily works the delts anyway). Another benefit is muscular endurance, a challenging set of 10 pwer cleans will leave you more spent than any set of shrugs or overhead presses ever would.

That said, to answer your initial question, I would go with shrugs every time over behind the neck press.

Have you tried shrugs holding a barbell behind your back? You can perform them in a Smith machine rack and lean away from the bar so your traps get an awesome stretch. Holding the barbell at a level just under your bottom, you lean forward (kind of like that skinny twit Leonardo Di Caprio at the front of the Titanic). Done correctly, you will defenitely feel those.

Good luck, GTForce