Without question, the Diamond version 804 is a better choice over the older 803s. The Diamond tweeter has a much smoother response and faster transient speed over the aluminum tweeters. This all translates (IMO) to a more realistic presentation right down into the midrange.
B&W 804D vs 803S
I currently have a pair of stand mounted B&W N805 speakers. While they sound great, I'm looking to upgrade to something with a bit more depth. I've tried pairing my N805's with a sub; however, I've never managed to get it quite right. That said, I have an opportunity to purchase a pair of brand new 803S's. However, I'm also looking at the new 804D's. I would say about 90% of listening is TV or movies and the other 10% on music. Right now, my setup is in the den of my house. 10 foot ceilings, open floorplan that opens to the kitchen and formal dining. Any advice would be greatly appreciated.