Canon 5D Mark III vs. 6D
The Canon 6D is the smaller and less expensive version of the 5D Mark III. They take the same battery and accessories.
The differences are minor, so if money matters, of course get the 6D for 40% less money. The 6D loses about 5% of some of the 5D Mark III features, most notably a simpler AF system, only two "C" modes, only one memory card and no auto brightness control for the LCD, but adds the same 5% back with GPS and WiFi not in the 5D Mark III. Many people will prefer the 6D for adding GPS with lighter weight.
They take the same pictures with essentially the same shutters, sensor, color controls, resolution and ISO range. Yes, there are many minor differences in the sensors and shutters as I'll outline below, but these are designed merely to help upsell innocent rich amateurs to the 5D Mark III; they aren't different enough for a real photographer or someone without a spare $1,400 lying around to blow on camera bodies to worry about.
The only missing feature that matters to me is that the 6D has only one card slot, not two. I use both as backup in my 5D Mark III which makes my backup plan much easier, since I can format my shooting card as soon as I download it to my Mac, and don't have to wait until my daily Mac backup to be sure I have two copies before formatting my shooting card. In other words, I always have a backup in my 5D Mark III's second card after I download and format, but no backup with the 6D until I download and backup my computer.
Top shutter and sync speeds are the same. No one uses 1/4,000, much less 1/8,000, so the 5D Mark III having 1/8,000 makes no difference in actual shooting.
Likewise, flash sync at 1/200 versus 1/180 is only one-sixth of a stop different, and specified this way merely as a sales tool to upsell rich people to the 5D Mark III. If the 5D Mark III was 1/500, it would matter, but even 1/250 vs. 1/180 is the same thing as far as I'm concerned.
The 6D adds GPS and WiFi, lacking in the 5D Mark III.
The 6D has a simpler AF system. You may prefer either; gearheads will prefer the 5D Mark III, and I prefer the 6D.
The MENU and INFO buttons pop out a bit more on the 6D because they are more rounded instead of flat, making them easier to find by feel on the 6D.
If you shoot all day with it as I do, the cost becomes less important and little things, like having a third "C" mode and second memory card, become more important, so I prefer the 5D Mark III.
Then again, if I have to carry it all day, the 6D wins on weight.
If you have to ask, just get the 6D. It gives a lot more for the money than the 5D Mark III.
See also Is It Worth It.
If you shoot every day with it, or shoot a lot of sports and action, then the 5D Mark III makes sense for its fancier autofocus system and 30% faster frame rate.
Technical image quality should be identically excellent. 20 versus 22 MP is invisible; either has more than twice as many pixels as anyone needs for anything.
The 6D improves on the 5D Mark III by putting the play, zoom and trash buttons where they belong: on the right side. The 6D adds up/down/left/right control to the big rear control dial, and thus eliminates the need for the old thumb nubbin.
The depth-of-field preview button remains properly on the same side as the grip, but is a smaller button at a less convenient angle than the big flat preview button of the 5D Mark III.
The 6D has no M-Fn button near the shutter, which I never use anyway.
The darkest light in which the meter works is an interesting bit of marketing trickery. The 5D Mark III is rated to meter down to LV 0 with an f/1.4 lens, while the 6D is rated one stop worse, only to LV 1, but that's with a one-stop slower f/1.8 lens. In other words, the meters are as good; but Canon tries to make the 6D look a little less good to try to upsell people to the 5D Mark III.
* Ignore this: if you wear out the shutter in a year it should be covered under warranty, and if you ever wear it out, replacing a shutter is usually only costs a couple of hundred dollars!
** JPG only. 5D Mark III also does thing in raw.
See my 2012 Full-Frame DSLR Comparison Table for more details.
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