What is more productive using a contractor who hires subcontractors or using independent skilled trade subcontractors?
What exactly do you mean by "productive?" Do you mean more efficient?
If so, the question becomes:
Who is more skilled at hiring and managing sub-contractors -- you or the GC you would hire?
If your GC is better at hiring/managing subs, I imagine it would be more productive to hire a GC. If you're better at hiring/managing subs, then I imagine it would be more efficient for you to do it yourself.
Of course, that is completely separate from the questions of which is more cost effective, which is more scalable, which would produce better results, which would cause fewer headaches, etc?