The EX503 IS, in effect, a replacement for the W-series.
We have put the 32" and 40" EX503 sets on display this morning and they look much nicer in the flesh. The Freeview HD is really good - we're receiving BBC HD Preview and ITV1 HD through the set's built-in tuner and it looks every bit as good as Sky HD.
Bravia Engine 3 is excellent, so why make unecessary changes when the budget can be spent on improving functionality of the internet related features etc. I'd also say the build quality is a step better than the outgoing W5500 sets as well. Things like the HDMI ports etc. have a much more solid feel to them than the older sets. There have also been improvements to the menu system and further new features such as the ambient sensor.
The real relevation for me, however, is the improved performance of the entry-level sets. The EX301 32" is a good bit better than the old S5500.