Depends upon what and how you shoot as to which model fits you, I think. I bought a 645E and have 3 lenses and had contemplated a change to another model then decided to keep it. My ONLY complaint with the 645E is the inabiliy to change the backs. But I rarely ever have to do that, so why spend the $$$?
On the other hand, if you're doing work that requires you to change out the backs often, the 645E isn't going to make you happy and you should consider the ProTL or the AFD. That's my 2 cents...which is about what my opinion is worth...
Got my 645E on Thursday -- a used mint model, still in the box for $475. I love it..great viewfinder, controls are easy to use, and so long as there are film inserts, the only reason I can think that someone would want removable backs is to use a digital back, anyway, or polaroid. I shot a test roll the same night and developed it -- no problems. I really like the aperture priority and the nice viewfinder with adjustable diopter!
Dans, there are actually three metered prisms for your 645:
The CdS finder with match needle metering.
The PD finder with match diode metering.
The AE finder with aperture preferred automation.
So, you still have options for your "old iron".
These are great system cameras, and built like a brick house.