This is, really, apples to oranges. 35mm has significant advantages over medium format in terms of flexibility, size, portability, lens selection (fast glass, zooms, specialized macro, tilt shifts, long telephotos, ultra-wides) high ISO, dedicated flash options, video ability and, above all, cost.
Medium format is, without question, the ultimate in image quality and if that is your sole criteria, that is the way you should go.
For just the price of a USED medium format back, however, you can acquire and have converted ALL of the Contax "N" lenses for your Canon. I am going to limit my discussion to these because they offer full auto compatibility with Canon bodies once converted by an outfit called Conurus (
www.conurus.com).
Currently, I have the Contax "N" 17-35 2.8, 24-85 3.5-4.5, and 85 1.4 all converted to Canon mount. They offer full autofocus and autoexposure and exif information. The person who does the conversions also includes the ability to microadjust focus at close, medium and far distances (Canon does not even offer that) and the ability to use the manual video controls. Focus speed is moderately slower and noisier than Canon lenses but certainly enough for everything but ultra-sports or similar.
However, I assume you are interested in the image quality of the glass as the primary factor. I have shot Contax 645 for many years. These lenses have the same "Zeiss look." People sometimes have a hard time explaining this, but, to me, it includes several distinct factors (1) vibrant color rendition showing an huge range of different tones and hues, but with distinct transitions and no bleeding or blurring (2) the best multicoating and flare control I have ever seen and beautiful "star" effects without haziness when point source lights are in the scene, (3) related to the flare control, incredible tonal range capable of showing and differentiating subtle tonal differtiations from dark to bright (4) wonder focus fall-off to a smooth, creamy bokeh (5) excellent correction for linear distortions (for example the 24-85 zeiss is far better than the 24-105L at 24mm and the 17-35 significantly better than the equivalent Canons) (6) outstanding sharpness through their aperture range and especially at prime apertures of f8 to f11 where the detail rendition exceeds my 5D2 sensor (17-35 noticeably sharper than the Canon equivalent).
People speak of the Zeiss "look" as "3-D" but they ofter don't explain. To me, this means that each element of the scene is distinctly rendered, in its own space, with clear differentiation of color and tones, crispness, and lack of veiling or blurring. This is a product of all the lens attributes put together. This is not an effect of in focus/out of focus transitions, as this "look" is evident even in fully sharp high depth of field scenes. The focus fall-off and bokeh only improve things when that effect is desired. Sure, there are lenses that may outshine the Zeiss in a particular aspect, but I have not seen lenses that, for my eye, put together the entire image "package" in the same way. Some people claim that they don't "see it." I certainly do, and I expect you would, too from your comments.
I will try to post some images to show what I mean, but don't have time right now.
By the way, the lenses are solid, hefty and beautifully made as are all Zeiss products. They should last many many years. They are not weather sealed, but I shot my Contax in all sorts of extreme outdoor conditions with no problem if you are even moderately careful.
I am currently having a 400mm f4 APO converted, and am looking to add the 50mm 1.4 and the 100 2.8 Makro, reputedly one of the finest 35mm macrox ever produced.
I personally am less inclined to purchase the Contax/Yashica glass just because of stop-down metering, no exif, manual focus only, etc. However, many people put up with these features to get the Zeiss look.
Also, if you are content with manual focus but otherwise want full compatibility, look at the "ZE" manual focus lenses in native EF mount. By all accounts they are superb and available in a variety of fixed focal lengths if zooms are not your thing.
I will not tell you that any of this will equal a medium format image in terms of ultimate quality, that's just a consequence of 35mm format. However, rumors abount that the next generation of 35mm might be 30 mp or more--a big jump. I will say that my investment in these lenses has been an amazing benefit. I am extremely content with my digital package for now and concentrate solely on my creatitivity, knowing that the lenses will deliver everything I expect.