Up until a couple of weeks ago I owned both a Phase One P45+ (and previously a Phase One P45) along with a Sinar eMotion75 LV (upgrade to Generation II is in the pipeline). I now own only the Sinar as I finish moving to the Hy6.
I can tell you from direct personal experience that David has hit the nail on the head when he refers to the priorities we place on the many factors that go into the choice of back. This is everything.
In my experience, if you want ultra-clean low-ISO files, the Phase does a better job. If you want deep shadow detail the Sinar does a better job. If you want the highest possible resolution, the Phase does a better job. If you want superior high ISO performance, the Sinar does a better job. I could go on...
In some cases, it is possible to change the rules and improve one back's performance significantly with extra work and post-processing. In many cases, it isn't.
The point to take away is which are the things you care about most in your image files, workflow or even handling of the back while shooting? Do you want these attributes "out of the box", or are you willing to do extra work to have them? I truly believe there is no "best" solution--if there were, we'd all have found it by now.
Once I get my Gen.II Sinar back, I'll look into posting some head-to-head comparisons of the P45+ and eMo75LV backs, using exactly the same glass, lighting conditions etc. if folks are interested. Please be patient, though--schedule is getting a bit crowded.
Kind regards,
Brad