I'm using a 503cw with a phase1 back, and with film. No issues there, but some of the old 'c' type lenses have worn sync contacts and so don't always trigger the back properly. 'Course, I bought the Hassey in order to USE the 'c' lenses, so I just get the sync trouble fixed. I looked at Contax, but decided the camera was too likely to die intestate, while a Hassey can be fixed any day.
The p45 has a 3/4 ratio sensor, a bit smaller than the film area, and it can be rotated on the 503 to change orientation. As mentioned, 6x6 film doesn't need to be rotated.
You can get TTL metering with a viewfinder add-on, and it's pretty accurate, but I've found it more of a PITA than spot metering, because, with the 'c' lenses at least, I still have to apply the readings manually.