If I may answer that, I have shot extensivly with the ZD back and now the Aptus 22.
The difference according to me is very well there.
The Aptus works MUCH faster in the studio, there is no buffer, meaning you can shoot tethered untill you run out of discspace, for me that's not really a problem because I only once or twice with the ZD ran into buffer full.
The ZD was quicker in continues mode with 1.2 frames per second instead of 1.2 seconds per frame, but again both are adequate for me.
Where the BIG gap lies is image quality.
The Leaf files (can't talk for phase) are MUCH more defined in the shadow areas and highlights (probarbly the 16 bits issue).
When looking at the design of the units the leaf and phase have a MUCH better display, although this is more of a status symbol because juding your photo on a digital display is a dissaster.
HOWEVER the composition and histogram is very good judgable on the leaf and was not usable on the ZD, the histogram clipped WAY before my raws would clip and the composition was off by about 10%.
But again, who judges their pictures on the display.
One advantage for the ZD was the removable IR filter which makes it possible to use the back for IR photography.
The disadvantage for this was/is dust.
I did not comment on this earlier because I wanted to make sure but the ZD is a real pain when it's dirty, it took me about one hour to clean the unit so there was only one spec of dust visable (when I got the back it was sprayed with dust), after that it kept clean for a LONG time, with the Leaf there is much more distance between the glass and the sensor meaning dust is not a big issue.
With the ZD because of the removable filter you have to first clean the sensor, than the back of the IR filter, connect the IR filter and clean the top, make a test shot and find out there is dust and repeat the procedure never knowing EXACTLY were the dust was/is, with the leaf there is only one surface.
Another advantage of the ZD was the option of a low pass filter which can be bought seperate.
Also the ZD back was VERY fast in boot and responded with the camera, meaning power up the camera and you are ready to shoot in 2-3 seconds with the leaf you are 7-8 seconds away from shooting.
HOWEVER it happend quite a few times the back did not shutdown when I shut down the camera meaning my battery was empty when I picked up the camera again the next day.
The leaf with tethered shooting does not need a battery, the ZD does.
The leaf has a rocksolid firewire connection which CANNOT be broken in normal use (or you have to use it as a lasso to catch running models maybe), the connection on the ZD is very brisk and not powered meaning you have to always use the battery.
Working with both will give you wonderful results.
I have shot several white papers with both the ZD and the leaf, and here it's really that you get what you pay for.
With the ZD there was some green infection on the bottom part of the sensor (about 10% too much green), the leaf is very very even, even with a picker the fall off is within the margin, if not you can use the gain files which I have never tried but it's good to know its there.
Longer exposures with noise control are on both systems not what it was designed for but the leaf does it better over 10 seconds below 6 I cannot see a difference to be honest (on ISO50).
The leaf goes down to ISO25 which is a wonderful thing for me.
The ZD goes to ISO400 with 1/3rd steps which can be usefull when you want to set things quick between 100 and 200.
On the other hand you can always underexpose 1/3rd step and correct it in PS.
ISO400 on the leaf is better than with the ZD.
Both are FOR ME usable in B&W but the grain structure (if we can call it that) on the Leaf is much more pleasing.
For color work ISO400 is for getting the shot but not for printing it large or customer use.
The ZD has AWB which the leaf does not have, but this is not necessary because the Leaf only shoots RAW, and the ZD also JPEG (which by the way in my opinion is not a standard I would set in a MFDB).
Crop factor is equal on both about 1.1 which is good enough in my book.
The price difference in the Netherlands is about EU 5000.00 between the Leaf and the ZD and in my opinion both give you a tremendous ammount of quality for the price, I have choosen the Leaf because I wanted 16 bits, I do alot of stuff that's on the edge of what my equiptment can take and I will always look for more so the Leaf was a natural choice.
When seeing the price difference of 15.000 US I would opt for the ZD without a doubt, that's just too much of a difference, unless you earn your money with photography.