The issue with night photography, at least working with star movements, is you really don't want to leave the back or camera open for 1 long exposure and instead your results will be greatly improved with stacking.
Stacking with MFD, is really next to impossible, (I can't say for sure if the Pentax 645z has an intervalometer), but Phase doesn't and Mamiya doesn't, so if you want to stack, you have to manually hit the release each time, and even with the 150/250, you will have gaps. Gaps can be fixed, (one way is to try the method I describe on this forum in the stacking article), there are others. Leaving open for 1 exposure costs you a lot faint stars. And if you work with the moon, which adds so much to the shot, you will be at F 6.3 to F8 and lose even more faint stars. This is just one person's opinion.
But even worse, if you stack, you need a fast lens, and there really is not a fast MF lens, i.e. good F2.8 wide lens by any company that I am aware other than the Mamiya 150mm F2.8, but that's way way to long for most night work. The average wide, say Mamiya 35mm F 3.5 wide open is next to terrible in the corners, enough so that your Phase 150 image will be cropped down to a 35mm sized image from a D810. The 35mm cameras, have some excellent wide and fast lenses.
Back to the crop, actually for some it's a big deal. Net it's 30% less capture space. Sure if you are in Death Valley or a lot of places out west, you can easily accommodate the crop, in fact it may work in your favor, just like a cropped 35mm sensor works for wildlife work. However in Arkansas and the Ozarks, I am not that often working vistas but even when I am, I will need the 28 Rodie to get the full shot. And when I working a creek, I only bring the 28 and 40, as the 60 and up won't work in most situations.
That's not to say, if Phase One came out with a trade in on the 260 to 250 that was not a total financial loss for me, that I would not consider it. I still need to shoot one with my setup, and see for myself and that's harder than most seem to realize, as getting a demo for me has been impossible. Phase is selling plenty of these backs to new users or upgrades from P45+ where they offer as much as 24K for trade in. Only time will tell if they decide to offer a more reasonable trade in to other backs, or if the market demands it. What a lot of people forget is Phase is selling a huge number of these backs in Asia probably more than 1/2 of their sales right now as that market is wide open. One of the reason's that the 50c was so cheap in Japan for a while.
As it sits right now:
Full frame, more resolution, needs more light, can't work well in low light if a faster shutter speed is required. May get to 15 minutes (260) with all the right conditions. Battery life is much shorter especially if you use Live View and or zero Latency or both. Back will heat up in 85 degree normal summertime shots and thus noise levels increase. Most times you need to bracket a series due to possibility of shadow noise issues. No real ability to push iso unless you use sensor plus, and STILL a huge debate on what the base iso really is 35/IQ180 or 100, and 50/160-260 or 100.
Cropped sensor, less resolution by 10MP (I can live with that), has great shadow recovery as much as 2.75 stops maybe more. Noise is less destructive more grain like and much less stuck pixels. Highlights may be a bit more tricky but now you can expose for hightlights and push shadows with ease. True step-able iso settings, up to 6400 (however from what I have seen of 6400 it not all that good for large prints), but if you do move from 100 to 200, you are stepping up the chip and changing the reads. Live View that works as any other CMOS back, and in fact possibly better as it seems to work exceptionally well in low light unlike NIkon or Canon. Don't forgot Alpa's post from over a year ago where they showed a 250 at night in full darkness and they still have a totally usable LiveView, I have yet to see anything like this from Canon or Nikon.
No doubt for a first time buyer, there is a lot more to consider now.
Paul