The reason the Yamaha is more popular is because the Pioneer VSX-1017 doesn't have audio or video processing over HDMI, which is incredibly foolish, IMO, to not have this feature included after running the last line the exact same way (HDMI switching, but no audio video processing).
The Yamaha can process high resolution 5.1 LPCM from a HD DVD or Blu-ray player. The power ratings between the two receivers is pretty neglegible as the wattage differences are only going to make or break about 2 dB and the Klipsch are pretty efficient and even loaded as far as Klipsch go.
Both of these receivers have preouts to facilitate a seperate power amplifier if more power is needed, both receivers offer auto calibration features, and both receivers have multizone capability. The THX certification doesn't mean a whole lot.