I think it's important to note that whereas both cameras are capable of 24P, the Sony F900 is an HD camera and the Panasonic SDX-900 is a standard def camera. Resolution is going to be dramatically different between the two. Comparing the F900 and the Panasonic Varicam is a little more "apples to apples" in the sense that they are both HD cameras.
That said, the SDX-900 is a great camera, and will give you a surprisingly good image. It's not the same as HD, but it produces a pretty picture. The quality of your image (as well as the cost) will ultimately be determined by the basics- your choices in lenses (with a Pro35 adapter, you can use 35mm film lenses), your lighting, framing, set design.
Regardless of your aquisition format, you can always choose to do a film out once the project is edited.
Meredith Seawright
MPS Studios