1) The D800 can bracket 9 frames. The canon 5DIII only 7. If you're doing HDR you may or may not need that. But you asked which is best and the answer is simply the D800, because I can where the other can't. You decide if that matters.
2) obviously resolution is a big plus. Not only is the D800 able to resolve more but you have the option of the E model to allow it to resolve extra detail that will get blurred by the 5DmkIII AA filter.
3) it's cheaper. Even the E model. You're getting 36MP and 9 stop ability and an non AA option for less. Is that a crazy good deal or what?
4) cropping. You can't always get that tree or pole out of the frame in the field. The D800 has huge cropping potential. You can crop 1/3 of your pixels and come out with a 5DMKIII file. IMO that's a big plus.
nobody should be surprised. The 800 is a landscape/studio/wedding camera. It is all about bing prints and superb detail. There was no way the canon camera, which is more of a low light high speed body, would match it in what it specializes.
As far as lenses, the debate is that 36MP is too much. Yet, that myth is debunked by the mere fact that the D800 has no higher pixel pitch than a D7000, which is a mere 16MP. Are lenses not good enough for the 7D? They are. So it is no issue, other than just fear mongering. Canon could make a 40+MP camera with a pixel pitch equal to the 7D, than lenses would be fine. It will off course mean that cheap bad lenses will show just how much worse they are than good glass. But nikon wides like the 14-24 f/2.8, 24 f/1.4, 24 TS, 16-35 f/4 IS won't even break a sweat as they easily outresolve the D7000 and at typical aperture landscapes of f/4-f/8, they are brually sharp.
Your technique will play a greater role than your glass. Camera shake and critical focus will become more important factors to consider. But if you're serious about landscapes, you should be thinking about that already.
I don't know that the extra ISO on the mkIII will offset the resolution trade off. If you're shooting your HDR at ISO 25K, then you're probably throwing several stops of dynamic range and should evaluate your lens choice. Depending on your workflow, that may or may not matter. Photomatix doesn't really use all the range in your RAW, but many people pre-process their files before going to photomatix and pull more DR than photomatix grabs before going to tone mapping. Based on my own experience in HDR sunset and sunrise, I don't think the cleaner high ISO images of the 5DIII will offset for what you're giving up. Yes HDR boosts noise, but you can deal with that in your workflow and IMO the D800 is really really good at 1600 ISO and just insane below that and having the option to downscale to 30 or 22MP is one you won't have with the 5DIII. But you be the judge as only you know what you shoot. 1 or 2 stops of light won't persuade me.
Now, I have a hard time believing the 5DIII is not suited for landscapes. If the 5DmkII was hailed as a landscaper dream, all the more the 5DIII. It just isn't the top dog in the block any more.