I took a special trip to Best Buy just for you Audioism to audition bookshelf speakers from both of these brands. I am certainly no audio wizard but I did have a few observations: (Oh and these speakers were both hooked up to Onkyo receivers)
The first thing I noticed about the Polks was the clarity of the soundtrack, especially at high frequency levels. I didn't seem to pick up as much ambient, high-frequency detail with the Klipsch bookshelves as the Polks. For example, with the Polk's I was able to distinguish the sound of the denim of an actor's jeans rubbing together as he walked down a hallway, while the Klipsch speakers seemed to mute out some of these more miniscule details. One of the best things about HD audio codecs is small details like this.
On the other hand, the Klipsch's had a warmer and more natural overall sound than the Polks. Comparatively speaking, music from the Klipsch speakers was silky smooth and heavenly to my ears, while the Polks (not bad by any means) sounded more "tinny."
I hope you find this somewhat useful. I enjoyed the research