First of all ignore what Brian says of Leaf, if you read on his website as review of e.g. Aptus 10 you can read of that he has not even bothered to get familiar with how to use the digital back mere than stick some San Disk cards that did not work etc. Instead do a search on internet of actual Leaf users who swear on the Leaf backs for durability, reliability and quality image. There was a recent thread on forum.getdpi.com of one back that fell into water, was dried out and still worked as should... my prior back dropped on hardwood floor once and bounced and rolled like a ball several times. Do not try it! Mere real stories...
In 2007 I had ZD camera, wich is same as ZD back except it is a camera in one unit. I sold it within six months due to a problem at the time, due to insufficient implementation of the sensor by Mamiya, and that caused problems. You can assume that Mamiya has not spent a fortune to fix it since, although I believe Capture One may be lessening the problem somewhat. I recommend to forget ZD.
On other hand, the Leaf Aptus 22 has the same identical sensor from Dalsa as in ZD, but Leaf implemented it very very well and correctly (and Leaf still makes digital backs, Mamiya do not, the now Mamiya branded backs are by Leaf). Many professionals have used the Aptus 22 back and it is as all Leaf Aptus solid reliable and delivers excellent image quality. I looked at it myself back in 2008 but instead purchased Leaf Aptus 65 which is 28MP but a slight smaller crop sensor measuring 44x33mm instead of 48x36mm. Big deal? Both are crops of 645, but because smaller crop it was at time lower priced than Aptus 75. It may still be that way...
. The Aptus 65 is one generation newer sensor and same sensor as Aptus 75, except the smaller crop.
Last year I upgraded my Aptus 65 to 80MP Leaf AFi-II 12 (same as Aptus-II 12 but for Hy6 camera). I am an advanced amateur and not rich. That was big money to me. However I did so with confidence in Leaf products and the support they give me. And no, I do not worry of San Disk card making problem (or I simply find out what is problem or toss the Sand Disk away and buy a new one...), I used/use a 32GB San Disk Extreme in both my Leafs, and unlike Brian I also use them extensive and learn how simple and solid they function...
When you look at Leaf do also look and read up on the display and what it offers you in large histogram and spot metering tool right on the display. I believe you will not be disappointed if you go with an Aptus, but advise to forget ZD. Per memory there was an ex user of ZD here on LuLa that the other week posted in some thread that he had upgraded to the Aptus 22 and was impressed by the improvements in image quality... The Leaf is worth any extra money. One more advise, do purchase from a Leaf dealer, and not on Ebay or a shop. That will give you support and likely also beyond warranty. They know what they are doing, at least mine. I upgraded with confidence after less than 4 years later... The sensor in a back is also the most expensive part, and you want to make sure to test it when/before you commit money.
Best of luck. Feel free to PM if you have any questions.