Hi Colin,
I recently (few months back) purchased a 5D III to compliment my 7D. I do birds and wildlife mostly, but I also like landscape and macro, as well as astrophotography. Short term, both cameras work well for the astrophotography, but in the long run, I'll need much more expensive and dedicated eqipment to do what I want on that front. For white field astro (i.e. milky way stuff), the 5D III is quite good, although a bit noisy.
For my birds and wildlife stuff, the 5D III is excellent. When I first got the camera, my 2x TC had been missplaced. I use the Canon EF 600mm f/4 L II lens for birds and wildlife. For wildlife, the 5D III at 600mm is excellent...it's just about the perfect combination. Deer, coyote, etc. at decent distances frame nicely, with a little bit of slight cropping and rotation room. Larger birds, like egrets, larger waterfowl, frame decently as well.
When it comes to smaller birds, including passerines (songbirds) and shorebirds, I have found that I feel a bit lacking in reach. I'm always wanting to get closer, and at 600mm much too close for most of those birds comfort. At 840mm things were better, but I still felt I wasn't getting the same number of pixels on subject as I did with my 7D. It was only after I found my 2x TC III, and started photographing the smaller birds at 1200mm f/8 at more comfortable distances that didn't influence the birds behavior, that I really felt I had the necessary reach to get the kinds of shots I wanted with a full frame camera.
For the more budget minded, I still think that for smaller birds, something like the Tamron or Sigma 150-600mm lenses on the 7D II are going to get you more pixels on target, which means more detail. I don't know how the 7D II will fare on the noise front...only time will tell us that. I can only hope that it's improved over the 7D and 70D IQ since it uses DIGIC 6, and if it has, I think it would be a better choice for reach-limited photography (at least until the 5D IV is out). If you have the skill to get close to frame smaller birds larger in the frame on the 5D III, then it should still produce better IQ than the 7D II, and at higher ISO settings.
The frame rate differences are another thing that I feel I've lost with the 5D III. Experientially, I know that the 7D's 8fps didn't always result in 8 keepers a second. It's AF system has an inherent jitter that tends to kill a couple frames each second, so in the end, you often effectively only get 6fps anyway. The literal 6fps on the 5D III does feel a bit slow in comparison...and it would feel very slow in comparison to 10fps. There isn't a huge difference in the moments and subject poses captured at 6fps relative to 8fps, but I think you could indeed capture better moments at 10fps, and that could be extremely useful for BIF...so that's something to consider.
Now, my preferred lenses for birds and wildlife are the EF 600/4 II and EF 300/2.8 II. I think that the 100-400 and probably the 150-600 (I haven't used the latter) are still excellent for the budget minded photographer. Ironically, I think the latter two lenses are still better paired with a 5D III and expending the effort to get closer to your subjects than a crop camera...simply because of the narrower maximum apertures.
When it comes to landscapes, this is the one area where I have personally been disappointed in the 5D III. I bought it partly for landscapes, as the 7D never gave me the field of view I really wanted with my 16-35mm lens. I don't get to do a lot of landscapes...all the beautiful ones are many hours drives away, and my life currently just doesn't support getting out there all that often. I've tried doing some landscapes recently, most of which had quite a bit of dynamic range...and I was rather disappointed in shadow noise quality of the 5D III. The 7D had vertical banding, however it was very regular and easily removed with Topaz DeNoise 5. My experience with the 5D III is that it's banding is much more random, and does not clean up as well or at all with DeNoise. The sheer amount of noise in the shadows is very high as well, and it's not very clean noise...it's banded, it's got a lot of blotchy color splotches in it, and it's more grainy and much more washed out than the random photon shot noise.
After the hype I had heard about improvements to the 5D III's shadow noise quality in the year after it's release, I figured it would have been better than it was. Certainly better than the 5D II. While some of the characteristics seem better, the overall levels of read noise are the same, and it is not any more "sightly" than Canon read noise has been for years. I love landscapes with water in them, which often precludes the use of HDR, or if you do use HDR, it complicates the merge. There are other options, manual blending, etc. that can be used, but it is an increase in workload that I can't really absorb, and even if I could, I just don't want to hassle with the added complexity. With more dynamic range and less read noise (as you get with cameras based on Exmor sensors, like the D810, D600, A7r), you can usually resort to less complicated post processing, and even reduced use of GND filters when on-scene, and just overall a lighter workload (something very appealing to me personally).
As such, I've been seriously considering an alternative for my landscapes, macro, etc. photography. Since I am rather heavily invested in Canon lenses, the Sony A7r is at the top of my list as an alternative full frame, high resolution camera with lots of DR that I could use for landscapes. It's not exactly cheap, but neither is it particularly expensive...and since it does have the potential for EF lens compatibility with the Metabones adapter, it's the cheapest alternative. The other alternative for getting the best IQ possible is the Nikon D810. I'd considered that as well, along with the 14-24mm f/2.8 lens...but the grand total cost on that came out close to $6000, which is just to pricey for me. The A7r is $2300, a much more reasonable price.
Your opinions on read noise and overall IQ may differ from mine, and you may find that the 5D III low ISO IQ is perfectly fine for your needs. If you think the IQ issues of Canon sensors might be an issue, then you may want to seriously consider an alternative package. The 7D II for all your reach-limited action stuff, and the A7r for your landscapes. The A7r is a mirrorless camera, so it's fairly small. The Metabones EF adapter is also quite small, so they can still be quite portable, even in combination with a 7D II.
