The EOS 5D provides three output image size options (12.7 MP, 6.7 MP and 4.2 MP) in combination with either of two JPEG compression settings (Fine or Normal). Additionally of course the 5D provides a RAW capture mode, written to Canons second version RAW format (.CR2).
You can combine the two in RAW+JPEG mode writing one JPEG (of any size/compression) and one RAW.
Below you will find 200% magnified crops of the same 240 x 100 portion of the center of a sequence of images taken at full size but different output settings. The RAW file was converted to a TIFF using Canon RAW Image Task 2.2 (via Zoombrowser EX).
You would be very hard pressed to notice any difference between the RAW converted image and JPEG Large/Fine, a relatively low compression level combined with a quality JPEG encoder. Even at the Normal setting artifacts don't really jump out of the image, you can detect some around fine detail if you look closely but it's safe to say that if your CF card is near full switching to Normal will gain you twice as many more shots with little downside.
The EOS 5D provides the independent selection of output color space, you can select from sRGB or Adobe RGB directly from the record menu (just above the selection for Picture Style). Images taken in the Adobe RGB color space have their filename prefixed with an underscore (_) this complies with DCF 2.0 (Exif 2.21), which makes it difficult to keep images in order if you shoot a mix.
Place your mouse over the label to see a ColorChecker chart shot in the respective mode. As you can see in this comparison rather than producing an identical result using Adobe RGB mode will deliver noticeably more vivid (and likely more accurate thanks to its wider gamut) color than sRGB. This is a bigger swing of saturation than we have seen from previous Canon digital SLRs.
Adobe RGB (converted to sRGB)
Color space: CIE u'v' Color Distribution chart
Note that in these samples the Adobe RGB image has not been converted to sRGB and so to view it correctly you will have to load it into a color space aware photo application and assign the Adobe RGB color space. Below each sample is the CIE u'v' Color Distribution chart; larger gray triangle approximately represents the range of color which the human eye can resolve, the inner triangle the available gamut in each color space (sRGB or Adobe RGB).