A comment from Karl:

After much consideration on the subject of moving from 35mm to medium
format for my main camera, I ended up going with Hasselblad. The Pentax
645 also was considered, but I felt that for the long haul, the
Hasselblad would hold its value, perform for many, many years, and that
the German-made Zeiss lenses are simply unbeatable. Then due to my
desire to do studio portraits (along with other types of photography) I
decided to get the 503 for it's ability to meter flash exposure off the
film. The 503 also allows for a wider range of accessories than the
501.
I couldn't be more pleased with my choice. From day one, I've loved the
way this camera feels in the hands, and the absolute precision of
operation and construction quality. I have experienced nothing but
great results, as long as I do my job as the photographer!