Bfisk hits the nail on the head...it depends on where you are with respect to the power curve. This argument is almost older than the Wright Brothers...years ago, I had a running feud with the chief at the college program I instructed at over it. "Aerodynamics for Naval Aviators" is adamant that pitch controls airspeed, and that makes sense when you are operating close to, or behind, the power curve (say during a carrier approach). But when I get slow in my 757 I sure ain't thinkin' of pitch, because I am well ahead of the power curve.
The problem arises most apparently in light aircraft, because many of them have little power margin. The power-control-airspeed approach works, but only within a small band of the approach speed range. If you get just a bit slow, there isn't enough power to get you back as quickly as your instructor might like. Naturally, this class of airplane is the one we are using to teach primary students, who have only an early comprehension of the issue. Thus, my preference was to use pitch, because it was more consistent, and it was guaranteed to be primary when the engine quit. But this should be accompanied by a thorough discussion of the relationships and an understanding that primary speed control would change from pitch to power as soon as you matriculate into a larger airplane.