While I don't like to disagree with 2F/2F....
The 645E is light and quite modern, and is much less likely to have been the main user body for a busy pro than an older '70s model.
The 645E has an excellent variable diopter eye level viewing system, with a very usable in camera meter, that doesn't suffer from the problems inherent in older CDS technology, or the somewhat common unwanted "line".
You can get a similar eye level viewing system for the Pro, with a slightly better meter, but it is fairly expensive.
The 645E works with that neat rapid winder grip.
Personally, I think the 645E is prettier
.
I will agree with 2F/2F that the Pro is even better. If you can swing one, a Pro Tl may be newer, and therefore slightly better still. If you have or intend to get one of the flashes that will work with the TTl function on the Pro Tl, it's better still.
I have 2 bodies - a Pro and a Super. I use the Super as a backup body.
I I had to choose between the Super and a 645E as a backup body, I would choose the 645E.
If I had to choose between the Super and one of the older bodies (say a 1000s) I would choose the Super.
Matt