The Canon 5d Mark III lists the options of All-I and IPB in the movie recording size menu so I dug around to find what these two options mean. All-I and IPB are different methods of compression, but both are still h.264 videos with a .mov output.
With this video compression, not all frames are kept and individually compressed. Instead, one key frame (or interframe) per every half second, for example, would be kept in full. For the other frames between key frames, the compression system just maintains the visible changes and uses the previous key frame to guess what the missing data is. The B in IPB stands for Bidirectional compression because this compression system looks at the previous key frame AND the following key frame to extrapolate the missing data. However, this compression method only allows in camera editing for one second increments. Further, frame by frame editing on the computer results in slightly lower image quality.
IPB compression is useful when long continuous recording is necessary but tight frame by frame editing is not required in post production. With IPB compression, a 4GB file would be roughly equivalent to 14 minutes of continuous recording at 1920 x 1080.
All-I uses an intraframe method of compression. This method treats each captured frame as key frames and compresses them. As such this compression method generates a higher quality and the capability to edit frame by frame. Because it retains so much more data, this compression method generates file sizes about 3 times larger than those with the IPB method. A 4GB file with ALL-I compression would generally only be 4.5 to 5 minutes of continuous video at full HD 1920 x 1080.
All-I would be ideal for short clips that need tight editing.