I had a good read today between the comparison between these 2 devices and here are my views : (Not seen in person, will do it today and update.)
1) Most Important part KDL-40W900A is discontinued. But yes you can still get it.
2) The design is almost similar, except the huge sony bump on 40" at the bottom which in my terms is a good thing for 42", it kind of distracted the view.
3) I read somewhere the blacks looks better on 40" model than 42" . There can be several reasons to it. But the advantage of 42" model here is "X-Tended Dynamic Range". What basically it does is what 40" model lacked. So let's discuss first what it lacked. Yes the colors were punchy and I loved those punchy colors but then for 40" it was difficult to distinguish between the darker and the lighter shades. It had very prominent Darker shades but when it came to lighter once it made them dark too. So maybe this was the reason the blacks were that black in 40" even if it was not supposed to be there. X-tended Dynamic range helps 42" to distinguish between lighter and darker shades.
3) Size : Yes 2" extra adds more value period.
4) Audio : I am an audiophile and never gave more weight to audio in LCD/LED panels. Where is the space dude for a good driver? You need a thinnest LED then a good Audio, ain't gonna happen, but both will give decent Audio o/p.
5) Miscellaneous: As 42" is the new model it gives a camera inbuilt, a cool new flick remote.
6) Price : So here there might be a fight over value and money. As 40" is a outgoing model of 2013, we can get it at a good bargain. In India without bargain I can get 40" @65k and 42" @ 70K. And I am sure I can get more discount on 40" one below 60 easily. The price difference is not huge so I would recommend 42" model.
I would like to hear views as I myself is planning to buy one.