Have you even compared the Phase and Sinar backs? I've used both and seen the raw files from both and would love to know what advantage you see in the Phase backs that would stop you from just getting the Hy6 today.
Assuming your question is not rhetorical, I can help here--as you may know, I had a P45 and a P45+ for about a year before switching over to a Sinar e75LV, which I am using now. The P45+ had a number of advantages for me as a nature photographer (no particular order):
* Lower noise at low ISO
* Multi-channel histogram
* Battery life
* Superior user interface (eg. one menu button)
* A unified file format (the Sinarback does not produce .DNGs, but rather .WR's, .BR's & .IA's, which must be combined by software to create .DNGs)--allowing one to go direct from camera to your choice of developing software, without having to create .dngs post-shoot.
* More mature software--Sinar eXposure has a number of limitations preventing a serious comparison to Capture One at this time, but Sinar is working on it. Anything from accessing content copied to a portable hard drive to disabling renaming so that the .DNG filenames correspond to the original camera-generated raw files and several other features are not yet ready.
* ~6% more resolution
* Able to shoot to card when tethered
* Custom WB when shooting to card
* Long exposure capabilities
This is not to say that the Sinar doesn't have its own set of advantages:
* Top-notch high ISO performance
* Excellent shadow detail rendering
* Interchangeable mount for near-universal compatibility
* Internal battery is secure
* Single-push CF card eject button
* Innovative internal memory feature
* eXposure produces a superior .TIFF develop from a e75LV .dng than does either Capture One or Adobe Lightroom 1.41 or 2.0
but it shouldn't be too hard to imagine why someone would see advantages in another brand. My point isn't that any one brand is the best (there is no such thing), it's that every brand has both advantages over and compromises relative to every other brand. Depending on how you work, the various advantages and disadvantages might amount to something significant, or they might not.