I currently shoot with the 5D and a 1DmkII. (Yes. I paid the $3299 back in 2005 ) I've never been let down by my 5D. it's always been a stellar camera, but I SO MUCH prefer the mkII in almost every respect except for resolution. I'm currently very very tempted to sell both bodies and buy the 1DmkIII. I enjoy full-frame very much but I don't necessarily need it. I'm quite used to and fond-of the 1.3x crop.
It's a tough call. But I can truthfully say the 5D still has amazing IQ in the most impossibly low light situations.
I just got my 1DMKIII yesterday. Aside from 1.3X crop, the image quality is on par with 5D. I used to have 1DMKII and wasn't satisfied with IQ then.
Side note, the 1DMKIII is much lighter than 1DMKII. The weight is about same as a 5D with battery grip loaded with 2 batteries. That's definitely a plus. Aside from wide angle requirement and shallower DOF, it is hard not to choose 1D III over 5D. Just my first hour with 1DMKIII, I only found the AWB works better with 5D under incident light. For manual focus, Live view just blows the 5D away. The 1DMKIII a better choice overall. As far as low light situation, I don't see 1DMKIII is any worst than 5D. The ISO 6400 is grainy buit definitely works when you needed.
They are both so different. I have two 5D's and a 1DMk3. The 5D is a fantastic camera for the price. I like it's smaller size and weight for use as my personal camera. On many trips I don't bring my 1DMk3. Now when my 1DsMk3 arrives in a week or so, then I will probably retire my 5D's. But I will miss the light weight and small size.
I have shot many 1000's of images with the 5D, Mk III and D3. The primary difference is at hi ISOs. I rate these cameras @ 1600, 3200 and 6400, respectively. At 800 ISO and less IQ is really pixel peeping at the 200% range.
However the ultimate IQ of the 5D seems to be a little better than the MK III and less so when compared to the D3. It really depends on the print size and how much you enlarge. For me the very small IQ difference is swamped by the ISO noise. YMMV.
To me I think the major advantage is it appears that the 5D has a weaker AA filter, because the difference in resolution is pretty significant for such a small jump in MP. It's much more noticeable between the 1D3 vs 5D, than a 5D vs 1Ds2.
They are both extremely excellent cameras (considering the 5D can hold it's own to a Camera much newer).
So I got a chance to shoot the 1DsMarkIII and the 1DMarkIII with an 85f1.2II and all I can say is WOW!!! As I keep mulling over the decision I think back to a lot of the advice I have given about lens over body. I think that for the price of a 1DMarkIII I can get a 5D and an 85f1.2II and still have $500 - $600 left over. Does that sound crazy?
Having shot 1DsmkI, 5D, 1DsmkII, 1DmkII, 1DmkIII, I would rate iq in the following (under ISO 800 and below):
1. 1DsmkII
2. 5D
3. 1DmkIII
4. 1DmkII
This is using the best of the best primes and L f/2.8 zooms. The difference for me comes into play with handling and speed. There is a difference when you need 8-10 fps vs 4 or 3 fps vs file size. Each camera was designed with different needs and requirements and this is where each shines in its own creation.
I really liked the lens, the focus speed was very good (wasn't sure what to expect with the 85LII as I read that the speed was better than the original) although I think that having a 1 series AF really helped. The menu system of both was just like using my 40D. The size and handling and balance was very good, almost making me think to add a battery grip to to my 40D to make portrait orientation more comfortable. AF was good although I didn't have a lot of time to experiment with selecting focus points individually, but not having the diamond pattern in the viewfinder I think would take a little getting used to - not bad, just different. The images also looked very good, the IQ was very good.
) I've never been let down by my 5D. it's always been a stellar camera, but I SO MUCH prefer the mkII in almost every respect except for resolution. I'm currently very very tempted to sell both bodies and buy the 1DmkIII. I enjoy full-frame very much but I don't necessarily need it. I'm quite used to and fond-of the 1.3x crop.