645D lost the competition

Also remember JPEG is a high compression format and only has 8 bits, so the high bit depth mainly benefits RAW shooters who process and edit in RAW format. If you convert a RAW photo file to JPEG and edit in Photoshop, you also lost the advantage.

The bit different will lay an impact on a very large scale print out , such as billboard or so. It's very helpful for professional who demand a large output of optimum details. The largest printout I've ever seen around is around A1-A2, originated by a TIFF file. Today printing technology doesn't really catch up with the monitor and Digital camera. What you see on a calibrated screen Mac doesn't always end up the same on most printers & plotters. Leave the work to the professionals who know what they are doing.

I've tested a friend's H3D2-50 and really disappointed for the price versus quality. At first glance, any shot ISO above 100 are extremely noisy like DSLR's ISO above 800. But, the dynamic range and details are something else, provided the correct lighting. Each camera serves its user purposes. To me, shooting pictures are a lot more fun to do. An average DSLR camera is more than enough for non-demanding and picky shooter. I'd be more concerned for what I did wrong for any shot. Enjoy photographing.

Agree with jump. Every type of camera has its purpose and optimal use. The D-SLR/35mm format is for photojournalistic and sports photography. It is portable and fast, with much better low ISO performance for low light actions. And this is also a great camera for candid shots of kids, on-location weddings and for traveling (especially if you don't want to carry a heavy tripod).

Here is an objective test of the high ISO performance of different cameras, and you will notice that all the high ISO kings are D-SLR 35mm, not the medium format cameras. In fact, the highly regarded Hasselblad ranks 63 in high ISO performance (beaten even by the entry level D-SLR with much smaller APS-C size sensor):
http://www.dxomark.com/index.php/eng/DxOMark-Sensor

Medium format has larger film or sensor, so it has wider angle of view and looks sharper (becauase you don't need to enlarge as much as the small 35mm format). It is good for landscape (large format is even be better) and studio work (photos with a lot of details). Upgrading from 35mm to 645 format is like upgrading from APS-C to full 35mm frame, with a crop factor of about 1.6 to 1.7. So medium format lens has "wider angle". Since there is not as much magnification needed to produce the final print, details are sharper. Medium format lenses are usually not as fast as the 35mm, and they don't really need to because no one uses medium format to do low light action photos.

As many people have said in other forums, camera and equipment are tools for the photographer. Choosing the right kind of equipment is as important as having good skills and knowledge.

Finally, one thing to add about Pentax vs Mamiya medium format. I did mention Pentax is cheaper, but Mamiya has more lens selection and accessories. If your budget is not limited, then Mamiya has more options. Remember lens selection for medium format is limited to start with, so having a system with more lens and accessory choices can be very helpful.

The point is when you buy a camera system, don't just look at the specs and certain tested performance. You have to look at what you need and how to get everything without going over your budget too much. Getting everything you need is often more important than getting the best individual camera or individual lens, etc. For medium format photography, if you plan to do studio type work, then you also need to invest in a good lighting system, because you will need it.