I may be wrong here but I'm pretty sure the phase one, as the branded mamiya, is the afdIII with another name..the new features of the AFDIII over the II can be found on the mamiya website.
I remember reading somewhere, for example, that anyone buying a AFDII with phase one back now will get free upgrade to phase camera / III when it is out...but don't quote me on that!
For waist level finder users the 645 mamiya or varient of will never be right, although there is talk of a brand new phase/mamiya body in the near future.
There are also some new lenses being made..such as 45-90mm as well of course the ability to use hasselblad v lenses via an included adapter.
Marc