"My photographic interests range from landscapes to macros to travel."
If those are you primary uses, IMO 6D would likely be just fine... Put the $1500 savings toward great lenses to use on it. You didn't mention what lenses you have with your 40D, and to get the best out of the new camera you might need some upgrades in that area, too. Or you might currently use some "crop only" lenses that would definitely have to be replaced.
If sports/action are key interests to you, then the 5DIII would be a better choice, largely for it's more sophisticated AF system. Also to some degree for it's faster frame rate potential. (However, see below for additional comments regarding FF vs crop.)
If your main interest is higher ISO shooting, a full frame camera can give you that. And 6D is one of the best, some even say slightly better than 5DIII. In-camera JPEGs, in particular, offer pretty amazing high ISO image quality. With RAW images, with either 6D or 5DIII there doesn't appear to be a whole lot gained over 5DII. That tells me that a lot of the gains are software based, and some additional work on RAW should be able to equal the gains, even with an older camera.
Just to put it in context... We all have different opinions about what's usable and what's not. So I can only compare my own experiences. When I was shooting with 10D, I tried not to exceed ISO 800. With 30D, I'd use 1600 but knew it would require some extra post-processing. Later I upgraded to 50D and 1600 was fully usable (I'd rate 40D about the same). Now with 7D, I'll use everything up to 1600 is quite clean, 3200 is usable, sometimes even 6400, with a bit of extra post-processing. By comparison, 5D classic topped out at 3200, but it was always fully usable. 5DII I'll use at 6400 without much concern. I figure 6D and 5DIII are a little better when shooting RAW (not even a stop's worth), but can pretty confidently be used at ISO 12800 or even 25600 if shooting in-camera JPEGs.
With any of them, it's critical to avoid underexposure to keep high ISO noise to a minimum. You never want to have to increase exposure in post-processing, or you will really amplify noise at the same time. In fact, it's better to have to pull back an image a little, to slightly overexpose and then reduce exposure in post-processing, to minimize noise. Softwares have improved a lot the last few years, too. I used to use Canon DPP for high ISO shots, it seemed to give the best noise handling but has never been up to the high volume image processing I needed to do. But it dealt with noise better than Lightroom up through version 2 and Photoshop up through CS4. Beginning with LR3 and CS5, though, Adobe seemed to figure out much better noise reduction. For particularly high ISO shots (and accidentally underexposed), I've been recommended and am currently experimenting with a Noiseware plug-in (also available as a stand-alone, if not using Photoshop).
To complement the fact that they can handle noise well at higher ISOs, both 5DIII and 6D have improved low light autofocus capabilities. I believe Canon rates them both to -3EV, which is approx. moonlight (note: on 6D it's rated this low at the center AF point only). In comparison, 5DII, 7D, 60D I think are all rated to -0.5EV... more than two stop brighter (note: I think this is a little conservative rating for 5DII... I know mine can still focus, albeit rather slowly, in lower light after my 7Ds have pretty much given up).
So, either FF camera will handle high ISO shooting better than your current camera. But, in general, a full frame camera will cost more to buy and to operate than a crop sensor camera. Plus you'll have a somewhat reduced choice of lenses (and might be forced to replace lenses you use now). And coming from a 10MP crop camera there might be other, unforeseen costs: You may need newer software, bigger hard drives for more storage, more and/or larger memory cards, etc.. With 6D you'll have to switch to SD memory, anyway. Fortunately it's not too expensive. 5DIII has dual slots and can use either SD or CF memory (or both).
It's the latest rage, switching from crop to FF cameras. But if you don't ever print larger than, say, 13x19" (or make equally heavy crops), you really aren't likely to see very much difference between FF and crop camera images. You certainly won't see any significant difference with images displayed online, at Internet resolutions and sizes. But, if you print big... really big, such as 18x24" and larger... full frame cameras will definitely have the edge.
Someone shooting mostly sports/action especially, seldom landscape/portraits/macros, may want to stick with a crop sensor camera. There are some other advantages to crop cameras, such as a smaller/lighter and less expensive lens kit. The 7D is now close to a four year old model, but still quite capable and a considerable upgrade from a 40D (5DIII and 1DX AF system is sort of based upon 7D's, but with even more AF points). The new 70D coming in mid-August to early September inherits much of the 7D's AF system, plus boasts a new sensor and a considerable bump up in frame rate from 60D, should be a very nice camera for sports/action shooting. I'm anxious to see how it performs in general, but especially at higher ISOs. In fact, it's going to have a considerably more advanced AF system than the 6D. Personally, though, I'm probably going to wait and see what the 7D "Mark II" looks like. The 7D has been such a success, I have little doubt that Canon will be rolling out a Mark II in the next 6 months or so. I hope so, the pair of 7Ds I've been using for the past three years have given me very good results.
So, if it were me...
- For lots of sports/action shooting, I'd probably stick with a crop camera. For that matter, if I rarely needed to use super high ISOs or make really big prints, I might stick with a cropper anyway.
If full frame still seems to be the best way to go...
- For mostly One Shot shooting (landscapes, portraits, macro), I'd consider the 6D and keep $1500 in my pocket or use it towards other things.
- But if a significant amount of my shooting were AI Servo (sports/action/wildlife), the 5DIII would probably make more sense.
Have fun shopping!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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