The D800 may be better at low iso - but is it going to be so much better that you could tell on a 36 x 24?
If not - is it relevant?
resolution-wise, not so much at normal viewing distances altho would be nice to have more real captured pixels to print.
For most mainstream kind of images, there is no big advantage to using the D800 IMO, even at 36x24" prints. I'd honestly prefer to use my Canon's for familiarity sake and the glass I have access to.
BUT.. when it comes to some of the high DR range scenes I shoot - AB so freakin' LUTELY!
The D800's lower noise low ISO files will allow me to push dark areas up to the levels I want without having to concern myself about the noise becoming a visual distraction. And I'd say that's an issue even with small prints like 18x12" or even down to 12x8" depending on camera (banding) and processing factors.
You can see some examples of the critical areas I'd run into as I described in this post last nite;www.canonrumors.com/forum/index.php/topic,5101.0.html
And that's not even a high DR scene. I'm thinking more about the storm systems I like to shoot in fast changing lighting. I have to allow a stop or more at the top end to not blow out textural detail on a suddenly sunlit bit of cloud, leaving me less to work with at the dark end. Any body that can give me clean shadows under these conditions that I can process up later, without clipping the highlites to get it, is the tool I want to use for this kind of shot. My old 40D could do this, none of the semi-pro Canon bodies I've used since do it as nicely. A new little D5100 is going to get a workout this spring in this very kind of shooting. If it does well, it's big bro could get ordered.
If I'm takin' pictures of the old folks or some event, this kind of extreme post-processing is not required or wanted, thus minimizing the impact of a technically better (at low iso) sensor.
The technically better sensor is of value to those few of us who push the limits of said sensors, and post-processing.
And multi-exposure HDR is not always a viable option when you're in the middle of some fast weather, the scenery isn't sitting still.