I am looking at the different Phase One bodies the 645DF and the 645DF+. What is the difference between the two bodies. I can get a pretty cheap 645df body but not a 645DF+. Someone at Samy's said the 645DF+ has more compatibility with the backs than the DF body. I am just wondering how much of a difference the shooting experience is between the two. Is it a huge difference or just a few improvements that won't justify the DF+ higher price tag. Thanks!