For a camera that you will handhold under circumstances that allow you to take your time and compose as you want then I believe you have to try the cameras to get a feel for them. If weight or bulk is a big issue for you then the lighter or more compact of the two is a definite consideration. If you are going to use them at eye level then you need to evaluate them where both cameras have a prism. This changes weight and has price consideration also...if I remeber correctly the Pentax prism will show only 85% of the image that the camera will caputure. If you are going to use the camera with on camera flash then the Hasselblad wins hands down since the flash stay in one spot and of course synchs at all speeds.. If you are going to use them on a tripod then again ther Hassleblad is much easier to use because it does not require any change from horizontal to vertical composition. Changing from horizontal to vertical composition is a PITA wih any camera on a tripod. This is where a Mamiya RB-RZ really shines. Perhaps you should also consider these cameras.
It is helpful to think a bit beyond how you antcipate using the camera and also to consider what else may be of some future importance.