Don't get me wrong, I love my 5D and I don't need the video features. That why I am still thinking about upgrade to Mark II (and never did). Should I upgrade to Mark III then? or Should I just save some money by upgrade to Mark II? ... Here what I found out:
|5D||Mark II||Mark III|
|year||Aug 2005||Sept 2008||Mar 2012|
|Processor||Digic II||Digic 4||Digic 5|
|ISO (Auto)||100-1600 in 1/3 stops||100-6400 in 1/3 stops||100-25600 in 1/3 stops|
|(manual)||50, 3200||50, 12800 and 51200||50, 51200 and 102400|
|Auto focus points||9||9||61|
|Continuous Drive||3 fps (60 JPEG, 17 RAW)||3.9 fps (78 JPEG, 13 RAW)||6 fps|
|Exposure Compensation||±2 EV (at 1/3 EV, 1/2 EV steps)||±2 EV (at 1/3 EV, 1/2 EV steps)||±5 EV (at 1/3 EV, 1/2 EV steps)|
|AE Bracketing||±2 (3 frames at 1/3 EV, 1/2 EV steps)||±2 (3 frames at 1/3 EV, 1/2 EV steps)||±3 (2, 3, 5, 7 frames at 1/3 EV, 1/2 EV steps)|
|Muli-exposure||no||no||up to 9 exposures|
|HDR in cameras||no||no||5 options|
|Horizon level||no||no||dual axis leveling|
|Memory Card||1 CF||1 CF||1 CF and 1 SD|
|Longest clip||n/a||12 mins (approx)||30 mins|
|Microphone/ Headphone Jack||n/a||mono/no||mono/yes|
|Audio level ctrl||n/a||yes (pre recording)||yes (while recording)|
|Aperture control in video||n/a||no||yes|
|Histogram in video||n/a||no||yes|
|Chromatic Aberration fix||n/a||no||yes|
|Time code support||n/a||no||yes|
Overall, Mark III is an improvement over Mark II in the photography featrues, little bit bigger censor, faster processor, wider range ISO, more auto focus points, two memory cards, etc. Mark III is greatly improve on HDR support, such as, HDR in cameras, AE Bracketing and Multi-exposure, and 6 fps would help to get better HDR pictures. For me, I wish to have 5 or 7 frame AE Bracketing for quite some time.
Beside HDR, I would say, other improved features are nice the have. It's won't convince me to pay almost double to get Mark III.
Obviously the different between Mark II and Mark III is in video features. I don't know much about it but I can see that Mark III is greatly improve over Mark II. If the video features are important to you, then you should consider Mark III.
On the other hand, if you are looking for your first canon full frame camera, spend some time looking at Mark II, specially at this price. It is a great photography tools and it is must better than 5D. It come with video features that you can explore. If I am not mistake, Mark II is very successful in this area too.
For me, I already have 5D. I am still happy with it and I don't really need video features. So I probably wait and see how Mark IV is going to look like.