Andrew over at eoshd says to use A-I, even though his first post said he saw a fizz noise at 160 iso, which i don't see on my camera,these are his setting suggestions:
Movie rec. size: 1920 24/25p, ALL I
High ISO speed NR – OFF
Highlight tone priority (HTC, D+) – OFF
Auto Lighting Optimiser – OFF
In Custom Controls set the SET button to Mag/Reduce for your focus assist
but another person says this:
"(I work with these codecs at the software level in my day job). My low level analysis of video frames shows artifacts present in ALL-I but not in IPB (part of the issue is PPro CS5.5.2). Thus, my findings show IPB is higher quality vs. ALL-I (especially lower noise, and less macroblock artifacts). ALL-I is useful for editing on slower computers; IPB provides higher quality (please post images from video frames if you find otherwise). I understand it's counter-intuitive, however I have tested it. You too can test it. More info here: http://www.dvxuser.com/V6/showthread.php?279229-Canon-5D-Mark-III-IPB-contains-more-detail-and-has-less-artifacts-than-ALL-I/page3
but everything on the internet i see that says this, actually comes from him, so i don't know whether to believe it or not, I've already done two professional shoots in A-I mode and all seemed good, but if IPB is better, which canon says the opposite, then id love to know other peoples opinion.
and canon says this:
"The edit friendly intraframe ALl-I only compresses information in the current frame and does not use any temporal processing. Meaning the compression algorithm is not doing any type of comparison between frames. Think of it as a continuous series of still images that are each individually compressed. Intraframe compression is easier to edit with because the computer does not need to interpolate any data between each frame. With intraframe ALL-I, quality is higher, file size is larger, and the video files will use less computer processing power.
The file size conscious intraframe IPB uses some complex algorithms to compare neighboring frames and tries to find similarities from one frame to another. It can then achieve higher compression rates because it deals less with the parts of the image that stay the same from frame to frame. With interframe IPB, quality is lower (although Canon says not by much), file size is smaller, and the video files will use more computer processing power."