I've been loving my 5D for over 2 years and then I added 1DIII. Now 1DIII is my primary camera while 5D remains a backup one.
Events, concerts low light applications and fast actions (using AI servo) are much better served for me with 1DIII due to its design and functions (not IQ itself). In the studio with studio lighting and ISO at 100 both cameras produce almost equal quality with the slight favor to 5D due to larger sensor and detail but it's only very slight and visible only at some specific subjects - like close-ups of cloths with material patterns detail.
Even though due to getting used to 1DIII I still use it in the studio as the difference in IQ is unimportant at this level while 1DIII has much better layout of AF points and I prefer more info in VF as it is in 1DIII. The AF points itself are larger in 1DIII then those in 5D and sometimes it's harder to select some specific contrast point (like an eye) as it covers half of the face
With 1DIII I don't leave it anymore with previous ISO setting as I see it now in the VF while with 5D I did it couple of times and realized this after seeing results on LCD.
The crop factor is only important when I do WA shots and then 5D is worth this extra WA - but it's rarely in my case.
Since I acquired 1DIII (last September) I only did some 1000 shots with my 5D and almost 7000 with 1DIII.
My current 1DIII blue dot (I exchanged the body for blue dot one after the fix was announced) is very good in terms of AF including AI. Every time I use AI I expect to see some OOFs and I'm always very surprised how few of them I get. I would say it's at its 90% of accuracy even when my active AF point is almost off the subject or goes away of it for a moment.
My only complaint and big disappointment with 1DIII is the resolution and quality of LCD. it's simply useless in terms of judgment of either the sharpness or the IQ. When I saw one at my friends Nikon D300 I was shocked how better it was comparing to 1DIII.
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Bartek
