The HDX900 is a 2/3″ camera which can be a great help or a great hindrance, depanding on if you want a narrow or a wide DOF. It has 1280×720 CCDs, so if your deliverable isn’t 720p, you’ll be working with uprezzed footage.
The XDCAM-HD you’re referring to is probably a CineAlta F-350, and some people have even suggested it keys better than a VariCam. I’ll stay out of that debate, because I haven’t tested them side-by-side.
What is clear is that you cannot compare the efficiency of an intraframe codec such as DVCPROHD with a GOP codec as used in XDCAM-HD.
In a lot of situations, footage shot with the XDCAM-HD 35 Mbps HQ codec can be considered to be less compressed than the same footage shot with 100 Mbps DVCPROHD.
There are exceptions to this, but the math is very simple and our eyes can confirm it.
My recommendation: rent both for a day, or worst case do a simple test at the rental company and bring some footage back to look at in your own NLE/compositor.
Personally, I hate working with cameras I don’t know inside out. Better to use a camera you really know than some other new camera that is “10% better” but with many unknowns.