it sounds like you are shopping along the same lines as me. I was jumping in-between a KV-36HS420 (no tuner), a KD-34XBR960 (nice nice nice), a KV-40XBR800 (too expensive), the KD-36XS955 (4:3) or KD-34XS955(16:9), as well as checking out similarly-priced RPTVs.
I found the direct views to have superior image quality (even after playing with the settings in-store) to any of the RPTVs (angle of view problems mainly). I compared the 34XS955 to a KD-34XBR960 and I couldn't discern much of a difference (jumping in-between rooms, however). The XBR has some funky multi-picture ability as well as more extensive DRC adjustments for standard-def. Other than that, the menus seemed exactly the same to me, and picture quality was nearly similar. IMHO, I found the XS to be a fantastic set for the money, and the XBR did not seem worth an extra $500 TO ME. I can't wait to see a 36" example (there was none in store), and I can't wait to get it when the funds come in

Sony is offering a free stand with it at the moment too.....
Back to your specific sets though - It looks like you are comparing three different model years. Check on newer features like cable card, integrated tuners, sony mem stick, HDMI, etc if you want them. The XS is the newest. Also, Keep in mind if you are buying "open box" you are probably buying the floor model that has been running 24/7 on the shelf in the store for X-years.