Neither - Pioneer or Onkyo is where I would start.
As the previous poster said, a budget would be a good place to start.
One thing to note - Audio "power" is often touted, but rarely understood. For a good "theater experience" it's important to have a good low-end response. In that light, current is what delivers the "punch" people expect, and Sony, for example, is notably poor in this department.
Remember that Power = Current X Voltage, so basically, what Sony seems to do is ramp the voltage to make the numbers look good, all while delivering low current levels. This is where the old adage of weight comes in - heavier receivers tend to deliver more punch (like Onkyo's) due to much larger transformers and capacitors (ie, weight)...
Again, these are rules of thumb, and not outright laws. In the end, audio is VERY subjective, and you should use your ears to judge, not someone elses. I have had excellent reviews from people who bought Pioneer and Onkyo, which is why I suggested them. I have also had poor reviews from friends with Sonys - one of whom changed to the Onkyo and loves it over the Sony.
And, of course, after all this comes the talk about speakers. Again, listen and test. My favs at BB are the Klipsch speakers...