If I shoot a feature in progressive full HD and use Cineform Prospect HD to edit and render to Blue Ray or DVD... given the same lighting conditions how much of a difference can one expect in the image/audio quality in the final product between shooting with a HDW-750PC vs EX1?
You shouldn't expect any difference in audio quality because a feature production normally calls for separate audio recording.
The 750 uses 3 CCDs compared the CMOS chips of the EX1. I expect that could make a difference visually. You're also likely to get better latitude out of the 750,and I-frame only compression. A similar camera was used to shoot Star Wars: Episode II, so that should tell you something.
The EX1 actually records a higher definition signal, the full 1920 x 1080, whereas HDCAM only records 1440 x 1080. But, there are other factors which can be more critical than resolution to consider, and those strengths will lie with the 750.
Of course, all this is theoretical. I'd be surprised if anyone here has real experience with both cameras. Which means you should do your own tests for equipment capabilities as well as post work flow issues, and make the decision based on your own observations.