B&W 803D vs 803Di

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.

Budt. There was nothing wrong with the setup or the speakers in question. Sometimes one persons slight improvement is another ones vast improvement, but hey, you heard what you heard. I respect that. If you look at what B&W did as far as the upgrades with the new Diamond series, it's not hard to believe the speakers would sound similar. First, it appears there has been nothing done to the cabinets and Marlan inclosures for the midranges and tweeters except for some trim cosmetics. Tweeter has some minor mods mostly with more magnets in the motor structure and a different surround material for the dome. This was mainly to maintain flat frequency response at very high power levels. The midranges appear to be basically untouched even in regards to crossover points. 80 percent of the music you hear in a loudspeaker comes from the midrange. Bass drivers have modest changes also still using Roacell cones. The bigest improvement to me seems to be the higher quality caps which are in the high frequency section of the crossover only. The caps in the older "D" series were also very good and made by the same company called Mondorf. I know I've mentioned what seems like alot of changes but these all seem pretty minor, at least to me anyway. I did note some minor improvements to the sound, but once again not world beating to me. Just my opinion.