IMHO the advantages (other than cost) of the 645E are:
1) lighter in weight;
2) can use the thumb operated Rapid Winder; and
3) built in diopter correction in the finder eyepiece.

The biggest reason to want interchangeable screens is so that you can install a grid screen. The second reason - some screens are better for extreme closeup and other specialised purposes.

Interchangeable backs speak for themselves.

It is handy to be able to switch to a waistlevel finder if you are photographing tabletop or ground level scenes

The build quality of a ProTl is heavier and more durable

If it matters to you the TTL flash capability on the ProTl is good

Just a few observations from an owner of one "Pro" body and two "Super" bodies

Matt