Hi Vikas
First of all this is a difficult speaker to drive well & control so a quality, big amp will be preferred.
I presume u mean the difference between the NEW 803 D & the older 803 D. Both have the diamond tweeter as far as I remember. It was the 804 that was offered as 804 S (Alum tweeter) and 804 D (Diamond)
At local MRP the 803 at 7.5L seems a bit stiff as the int rates are around 9000/9500 USD
The speaker can sound a bit heavy if there are walls close and that clouds the rest of the performance. If u have enough room behind and on the sides and the room that they are to grace is LARGE then only should u consider them.
I owned a B&W for too long and have heard most avatars of their popular 800 series. In most of the ones with the alum tweeter U must be clear in your mind (after many an audition) that its not over the top & that this is a high frequency that u can live with. The diamond tweets are more refined and lack that 'extra' bite.
Fit & finish and back up in case u ever need is good. Its a good speaker and one that I do like but its a bit of modern HIFI signature which is the only reason I drifted away from B&W.
U must hear the 803 & the 804 (in case u have a not so large room or u are not a bass freak) and see if they appeal to u.
Rgds