B&W 803D vs 803Di

Thanks, Mark_nz. Given your answer, how would you adjust it if these were to be geared more for theater experience rather than music? Say, 90% movies, 10% music.

Assuming other considerations are equal (age, condition, finish preference etc), then for movies I think the savings will be better rewarded by upgrading the AV and projector.
The 803D/HTM2D is more than good enough for movies. However if you really very picky about voice quality, the 803/HTM2 Diamond may be sway your choice - recommend you compare the HTM2D and HTM2 Diamond.
Also for background music the 803D is very good; most of the improvements for the 803 Diamond will only be appreciated when listening in the sweet spot.

If you can go and have a listen. The newer range is vastly superior in my opinion.

I compared the B&W 802D against the 802 Diamond at a Hi-end dealer. Same room, same electronics, same music and loudness levels. From my experience these two systems sounded almost identical. Scary close IMO. Even the salesperson thought they sounded very much alike. While I didn't make a comparison of the 803D and 803 Diamond, I would have assumed a similar outcome.

If the old "D" series and the new diamond series sound the same or almost the same to you there is something very wrong with the setup. I have owned both and listened to both at a dealer. The new diamonds are vastly siperior. I even thought the 805 diamond was better than my ( then) 802D.