I've seen both, in room, at the same time. The HX850 does have slightly deeper blacks. However, there are a couple of caveats.
First, turning the Advanced Contrast Enhancer (I think) to low on the W900 made the blacks deeper with no discernible negative effects. (hat tip to one of the review sites that mentioned this)
Also, with the dimming option set to standard on both, the HX850 tended to make very dark scenes go to near black from time to time, while the W900 seems to get that right. 1 scene in particular that I tried this on was from the last Harry Potter movie. There's a scene where the 3 protagonists are having a conversation in a stairwell with one of their friends. The HX850 after a second of two would invariably make this scene almost completely black. Sony seems to have figured it out for the following year's set.
Lastly, and this might be tied to the previous point, the W900 had better blacks with the automatic dimming OFF--in other words, deeper blacks natively. This might explain why the W900's dimming algorithm is not as aggressive.
Now, just to let everyone know where I'm coming from on this, I was extremely upset when the replacement Sony sent me for the 850 was not a refurbished 850, but a brand new W900. My brother commented on never seeing anyone so unhappy to get a brand new TV before. lol. I was so happy with the 850 and had not heard the best things about what it was replaced with. So I called Sony before even turning the new set on to find out what options I had. And after first setting it up, my first thought was that the out of the box settings for OTA broadcasts were MUCH better on the W900. But then I hooked it up to my PC and was very disappointed, only to find out it was because I had missed the Light Sensor option being on.
All in all, 2 great sets, both enormous steps up from my XBR9. I'd say black levels go to the HX, no question. But it is really close. Colors, depth,pop and....consistency of black levels goes to the W. Not to mention text when displaying PC is better (yes, even with the setting set up correctly on both). To me, I think the W is a better set OVERALL, although I loved the HX.
Just my 2 cents.