Well, beside nice to have things like the tilting screen, what does the 645z offer as real big advantage? Live view is nice to have from my point of view, too as the focus indicator in my 645D has always been very precise.
Well, the dynamic range of the 645D is not bad and sufficient for many things, but for example when shooting into a sun rise or sun set, even if the sun is not on the frame, it sometimes is a bit tight and filters should be used. The 645z offers two stops more of dynamic range at low ISO, so it is replacing a 2 stop graduate filter. That's not doing the job alwa, but sometimes it is. And sometimes it is hard to use such a filter, because of an abnormal shape in the picture.
On top of that, the 645z offers much more high ISO. Which isn't that big advantage for a landscape photographer running his I camera at Base ISO on a tripod. But for portrait work, in case you hate using flashes, it is a huge advantage.
Now regarding the lens: The 28-45 offers finder details (often called more sharpness) and wider field of view. Now if the 35mm is alway wide enough for you, the wider filed of view is nice to have. But in case you are suffering right now because 35mm is not wide enough, there is nothing more to discuss, get the 28-45mm.
Now let's put those 28 vs. 35mm away for a moment. What would be an advantage of that lens? Well, it is most likely the sharpest lens you can get in this range. This is easily compensating for the 40 vs. 50 MP, as that is only roughly 10% more pixels in each direction. I bet the 28-45 on a 645D shows at least the same amount of details at the borders of the frame als the 35A oder 35 FA on the 645Z. Most likely 28-45 on the D is even slightly better at the borders.
Conclusion:
If suffering because 35mm is not wide enough, there is no question.
If the 28mm instead of 35mm is nice, but not crucial:
a) You like using a tripod for landscape photos and for your portrait work a flash if light becomes short: Get the 28-45, getting the sharpest lens you can get is more important than 25% more pixels
b) You like doing portrait work at low light without using the flash and you hate carrying around a tripod: Get the 645Z instead.
Last edited by donesteban; 02-09-2016 at 07:54 AM.