Keith, opinions vary on your good points. I looked at the two from a landscape point of view and a Canon Shooter since 2002. I tried to make the 5D MKIII work, but found the files too much like the 5D MKII in that the same issues carried over. Here are some talking points to consider.
Faster, I would tend to agree, the 5D MKIII is considerably faster, at 6 FPS (I think I have that right) vs the 3 the D800 can pull in raw mode. Not to mention the fact that you will hit the buffer pretty fast in continuous mode. I don't think the D800 gets 5 FPS even with the grip and in jpg mode as Nikon claims.
More responsive, Not sure on that, as to me the controls on both bodies are well placed (expect the zooms I still like them as where the 5D MKII had them on the right side). Both cameras seem pretty equal here. No shutter lag, quick and accurate AF, 5D MKIII's AF is amazing in low light, however I find the D800 can get the job done. I am not using it in a AI servo mode or in action photography however.
Better High ISO, I was not impressed at all with the 5D MKIII's high iso performance. I quickly compared it to my 5D MKII and really just didn't see too much difference in the iso 800 to 6400 range. I just don't shoot any higher and the 5D MKIII may do better above 6400. What concerned me was the fact that the 5d MKIII still showed the same banding, and noise issues that every Canon I have used did. Blue red noise in the shadows and banding. In all the higher iso's and even the shadows of iso 100. Here the D800 to me is a much better soluiton. As Wayne pointed out the DR of the D800 is the real champion, not the extra resolution. I would just as happy with 20mp or 36mp but the Dynamic range is really amazing. Also the D800 seems to have gotten a bad name on high iso. I find it very useable up to iso 3200. When comparisons are mode between the D800 and others, one has to either down rez the D800 file or uprez the other camera. This became very clear with the D4 vs D800. To the two cameras are very close up to even iso 4000. After that sure the D4 begins to show better results. If you take the D800 file and down rez it to the size of the D4 at 16mp then you see that the two files are very close indeed in relation to noise.
More versatile. Again to me this is a draw or I give the edge to Nikon. The ability to shoot in DX, FX and two other formats is great. In DX mode and still get 15mp to me is an excellent solution. Canon on the other hand offers the 3 raw sizes and I miss not having medium raw for night shooting. I don't use the video features but it does seem from what I have read the Canon 5D MKIII is more video friendly. Live view on Canon is to me easier to figure out, but once you start working with Nikon's version on the D800 you can see it's merits also. I will say now that for daylight shooting I prefer Nikon's method, but for night work, Nikon's solution is terrible. You can't work with it very well as the noise is not buffered out as well.
I guess it all boils down to one own shooting style. But what made me switch overnight was the Dynamic Range with Nikon and Lack of improvement to Canon's with the 5D MKIII.
Paul
