Hi Gavin. You have come to the right place my friend for I use both... and I shoot weddings and portraits. To see this, look in the Contax gallery under Fotografz and scroll to my wedding stuff. Also have stuff posted on Dirk's Canon Gallery.
First off, a lot is dependent on what your wife want's to accomplish as she moves to digital.
If it is handling speed, working in higher ISO ranges and using super-wide lenses, then the Canon 1DsMKII is it. Why? Because the Canon AF is lightening fast, the sensor is a full frame 35mm so there is no X lens factor to effect wide angle lenses, and the newer digital offerings from Canon perform well at ISOs above 400.... in many cases as good as high speed 35mm films.
If it is image quality, the Kodak back on a Contax 645 still out performs the Canon. Why? While they are both 16 meg sensors, the Kodak sensor is a CCD and it is larger... which produces smoother tonal gradations. (same concept as film, more real estate wins). BTW, the Kodak sensor is square, which eliminates the need to turn the 645 on it's side. You can select square, portrait crop or landscape crop in the menu and never need to turn the camera on it's side ... which means a flash bracket is no longer necessary. Because the sensor is square there is a X factor that magnifies the effective focal length of each lens making it a bit longer. Then there is the issue of Zeiss verses Canon L lens performance.... which in my opinion is no contest except for a few L Image Stabilized (IS) lenses in the longer focal lengths. IMO, the Canon W/As leave a lot to be desired. Top ISO for the Kodak back is 400, but I often use exposure compensation to extend that to 800.
One other thought to consider. The 1DsMKII is digital, period. To shoot film will require a second camera like the EOS 1V if the same performance level of the 1DsMKII is desired (I have the 1V camera also). On the other hand, the Contax 645 is both. Switch backs, and you can use high speed film or B&W films at will.
Unless the Leaf back is totally portable, meaning no tether line to a separate image bank is needed, then the Kodak back is a better choice for weddings. Why? Because the Kodak 645C is entirely portable with an on camera LCD screen, use of CF cards for capture, and clip on batteries that last forever. The Leaf is 6 meg, and in most cases the Canon will produce better image quality.
Here is an offer made in heaven for you. Get her both !!!
I am moving to a 22 meg digital back for my commercial studio work with the 645. It costs an arm and two legs ...plus part of my rear end ( like 30K ! )
So, I am selling my Kodak 645C back complete in the box with all cords, programs etc., plus 4 additional batteries, four 1 gig IBM micro-drive CF cards, 2 chargers ... AND a 8 meg Canon 1DMKII. Quality of image, plus speed with the 8 fps, 40 frame buffer, Canon 1DMKII (much faster camera than the 1DsMKII). I have a Canon 550EX flash/Stroboframe Pro-T Quick Release bracket wedding set-up that can go with that. All you would need is a Canon 24-70/2.8 L to provide her with a complete wedding package for optimal 645 image quality with formals and portraits, and optimal speed for candid work with the 1DMKII (which, as my back-up camera is lightly used and in mint condition)... all for less than the 1DsMKII, one zoom lens and a flash.... $9,750. for all the above plus shipping to be exact.
Contact Marc at:
fotografz@comcast.net if interested.
Oh, and one other consideration, if you decide on a 1DsMKII, your wife will need a back-up camera. NO ONE who shoots weddings does so with just one camera. So add that to the expense of going Canon digital. With the set-up above either camera is capable of shooting an entire wedding in a pinch.