This has been a major discussion in my CFII training. Officially, the Instrument Flying Handbook says you can pick both and still be right. Personally, I like to have a good knowledge of both, but tend to prefer the control/performance method. Here's why: the primary/secondary instrument scan is a good idea, but they tell you what's happening either right now or what has already happened. The control/performance method goes hand in hand with the pitch + power = performance idea. Use your knowledge of the airplane and go with power settings and flight profiles to stay ahead of the airplane, THEN use the instrument scan to evaluate how that's working out for you.
For an instrument checkride, I would have been hard pressed to write a 2 page paper on the subject, and I think it's overkill for an instrument student to have to do so. CFII training, 2 pages would almost be too little.
