Not sure why you have found it difficult to find Aptus 22's in the Mamiya mount. In the last 4-8 weeks, they have been abound. Mpex.com has had 8-10 of these as cast off from one of the large photo schools. They are brand new, mine started with 0 shot count. They just have to sell them as demo status so that Leaf promotions (reduced extended warranty, free AFDII bodies with purchase) are not eligible for those purchases. They had one that just ended a few days ago that didn't sell. Call them up and get a "new" unit for a great price.
Here is the best little free app for the Mac that lets you keep track of any search you want to create for ebay auctions: http://www.iwascoding.com/GarageBuy
I have about 50-60 saved searches in this app and never go to ebay unless I need to. Saves a huge amount of time and you get constant updates. You have to be clever with search strings to get variations of how people post items for sale, but I have seen many digital backs on ebay for months in the AFD mount, Aptus 22, the highest.
I still agree that you should test for your style of shooting and user experience, but if you were originally interested in the ZD back and now looking for an Aptus, they are out there with no strings attached.
I commend Bob for posting some simple samples of his recent ZD back, because it does show that the file quality is there. Basically what I am seeing from my Aptus. The huge difference between the two backs comes down to speed, the screen and tethering.
LC11 for Leaf tethers flawlessly to my Macbook. Lightroom makes beautiful conversions of the Aptus files and the back itself on my AFD (non II) works with no errors or hiccups. I have 6 batteries, found a cheap source online for good L-ions and some faster CF cards.
I like easy to use items. I don't like to jump through hoops to use gear.
Caveat to the warranty for my back, it has a full one Leaf warranty which Leaf America would handle with very quick turnaround of a few days. But because of where I purchased the back, I can't expect 24/7 tech support as Steve Hendrix provides to his clients. But at the same time, I consider myself skilled enough at all the tech side, it shouldn't be too difficult to solve issues barring major service repairs.