A few comments...
Graphics Engine - as TVbc mentioned, the 52" version of the XBR7 uses the same engine as the XBR6 and the W-series. Sony really buggered up the XBR7 series when they made each size have a different set of features as some of the XBR7s have the next better graphics engine. Dumb, dumb, dumb. As far as the 8-bit color panel on the W-series and the 10-bit panel for the XBR6/XBR7... I don't see how this could have an effect on motion, but who knows.
Sounds - Two main reasons I didn't go with the W-series was the 8 vs 10 bit color panel, and the sound quality of the W-series. Agree with you zoro... the sound quality on the W-series is really bad. I didn't expect much better with the XBR6/7, but was pleasantly surprised. Unless you want surround sound, no need for auxiliary speakers in my opinion.
Price - Depending on who you want to buy from, and if you hit the right pricing window, the XBR7 can be cheaper than the XBR6. Go figure.
Warranty - Contrary to a lot of things I've read, all XBRs do NOT have an 18-month warranty. The 52" XBR6 only has a 12-month warranty. The 52" XBR7 has an 18-month warranty. If length of warranty is a concern to you, then make sure you read the actual warranty card BEFORE you purchase to make sure of what you're getting!
Another plus between the W-series and XBR6/7 is the remote. The W-series just comes with a remote that controls the TV. The XBR6/7 comes with a "universal" remote that can be programmed to control other items. I'd planned on getting a Logitech universal to control everything (and still might!), but for now I can control 99.9% of the functions of my Comcast HD set top box, Panasonic DVD/VHS player/recorder, and the TV from the standard remote that came with the XBR6/7. Nice.
Personally, for the 52" model I see a bunch of reasons to spend the extra $ to get a XBR6 over a W-series, but very few reasons to get a XBR7 over a XBR6.