My wife would disagree somewhat, Richard.
In the past couple of months I decided to forego the 1DxMkII's and soldier on with the MkI's and decided to have try with the Pentax 645Z. I also retired my 5D3 and picked up a 5DSR to fill its shoes.
So two different formats aimed at similar subject matter (landscape and portrait) and each clearly have their strong points and downsides...so here's my take.
Sensor.....the 645Z sensor would seem to compare favourably with the 1Dx as far as noise/iso is concerned....quite something for 51+Mp and well beyond the usable range of the 5DSR. Shadows can be recovered noisefree from 3 stops underexposed and a full 5 stops with a little NR work in PP.....it's quite remarkable. The following screenshots are a SOOC raw file underexposed by 5 shots (maybe a little more) and then simply pulled up in LR. The histogram really illustrates how little light the 645Z needs to generate a usable file.
There's so much latitude in the raws that ETTR isn't a prerequisite with the 645Z and it's just a case of staying away from clipping highlights...the shadows are of little concern. The DR from the 645Z is noticeably better than Canon although Nikon and Sony may well be closer and hence less potential gain for your £'s.
Weight......the 5DSR and a few lenses can be carried around all day whilst equipping the 645Z bag will have your shoulders aching. The 645 28-45 zoom is about the same size and weight as
a 35mm 70-200/2.8
Lenses.....the newer 645 Digital lenses are very much the equal of Canon's latest offerings when it comes to resolving detail but they run at 2-3x the price. I've used my Zeiss 135/2 (optically the best lens that I own) on the 5DSR and it seems to make good use of most of those pixels. Pentax have limited their zooms to 2x or less and perhaps Canon's best zoom offerings are in the same mould (16-35/4 and 11-24/4) which puts them closer to prime lens quality. It's feasible that the 35mm lenses are sharper but probably only by the extra enlargement factor needed to produce the same size print.
There's a wealth of cheap, older lenses for the 645 system and they perform fine if you're happy to shoot at f/8 or slower....simply for maximum sharpness
I live in SW France and the harsh summer sun results in deeper shadows and I often found it difficult to get a good, balanced exposure with my Canon setup.....I normally carry a couple of IR converted 5D2's to play with in the afternoon. The Pentax is far more adept in these conditions and has just coped exceedingly well with sunny, snow capped mountains and shadowy ravines all in the same shot.....not unlike HDR with a single exposure.
I intend to keep both for their positive points but it'll be the 645 that gets the most use with the 5DSR being the more portable option (and a chance to use the Zeiss lenses). I could go on and on but they're only my opinions and the circumstance might not fit everybody's case.
Bob
), get a Mamiya RB67 or RZ67, rotatable backs so you can go landscape or portrait without messing about with L grips or whatever. The negatives are nearly 5 times the area of "full frame", and a perfectly easy 2400 dpi scan will get you a 42 mp image. And you can choose the image size after the shoot, go for double that if it's worth it, or much smaller if it's one of the less successful ones. Drum scans would get you even more.
And I'm not sure the reality of leaf shutters is quite the Nirvana it looks like on paper - limited focal lengths, optical limitations imposed by the shutter, old lens designs, sync speeds less than claimed, and also limited by flash durations etc - so you might have to spend on flash anyway. 
