To me the most telling phrase is this is a big purchase for you, so my suggestion is be careful and think long and hard.
Since these are both demos, ask if there is a warranty. A demo doesn't mean they're perfect, it just means not sold.
Saving a few grand means nothing if you end up with a back that needs $3,000 in repairs, or turns into a paper weight.
Also take this you can't upgrade a Hasselblad camera (regardless of backs) with a very tiny grain of salt.
H1's, H2's are falling out of the trees for sale and in rentals and other than some DAC corrections and the 28mm lens, if you go to a non blad back on an H1/h2 it will do the same as any other medium format camera, actually more as it has a removable finder, leaf shutter lenses across the range.
There are probably more professional medium format images shot on an H series than all other camera makes combined.
If you upgrade the phase/Mamiya body it requires a brand new body like the DF to use the new leaf shutter lenses, so there isn't a lot of savings there.
Also I strongly suggest before purchasing, call a dealer for each of the brands your looking at and get quotes for repairs.
Phase has a $2,000 minimum (or something like that) for any repair and I think if something goes out on a Leaf they replace the electronic box, so get a price on that.
In regards to the Mamiya back it wasn't ready when it came out and few people sold theirs to upgrade, they sold them to get away from the problems.
I would really take a look around the marketplace, call some reputable dealers, preferably those that have competing brands, like Samy's in LA. (Call Karen in the pro dept.).
Or any brand specific dealer like capture integration and get the real cost, real numbers, real availability of what is out there.
Right now there are Hasselblad refurbished 39mpx camera kits at around 10k which may be past your budget but they are backed by a warranty and probably will last you a lifetime.
On this board there are two H series with Phase backs for around 6k and though I don't know the condition, I know my two phase backs have never gone in for any service and I use Contax which may be out of production, but works as well today as it did when it was new.
In fact I have 4 bodies, double lenses, 6 prisms, etc. etc. and very few times have ever gone to a backup and it was never a catastrophic problem, usually just a dirty contact on the lens.
So be careful and make sure your purchase will be rewarding long term.
IMO
BC