The move from the P45+ to the 160 for me was at first a tough call. I had spent 6 months getting my P45+ up to spec, (mine had the controller card that would not go to 1 hour, BTW a little known about issue). If you are looking for a use p45+ make sure you test it as I am sure there are more out there like mine. Since all warranties are off of the used backs now, it will be harder to get Phase to correct the problem.
For me, it's just the opposite. I used the P45+ for almost 4 years, used it hard. I quickly learned, expose for the highlights, and make sure you don't blow one as it's gone. Thus even at ISO 50, working with the P45+ was a mulit-shot show, most times 3. File size was not that big a deal so that was the way I went at it. I also noticed a big improvement in the 400 and 800 iso after Phase returned my back (new controller card that allowed me to run the most current firmware). But I still saw and still see when I work old files, what I call muddy blacks, and digital weirdness in the shadows. It's like the back was not able to figure out what to record in these parts of the image. Still when the exposure was good the results were top notch.
After a few demo's with Capture Integration with the P65+, I decided to trade the P45+ and upgrade. I also had read Don Libby's and Jack's posts on the P65+ and these helped a lot. Net, the DR of the P65+/160 is a huge step up from the P45+. It's there in the Dxomark scores, but I didn't really know how big it would be. You have so much more room between the shadows and highlights and now I often find it's just like a D800 file. Shoot for the middle you can recover a good bit of the highlights and the details in the shadows are just amazing. At first I wished I had done the P65+ a year earlier, but since I knew I also wanted a tech camera, I waited for the 160. Previous comments on the 2" LCD screen of the P45+ and P65+ are very true. It's a worthless screen and more times than not, when the screen showed me in focus, I would be out.
At first I missed the 1 hour reach. But in reality, for me, it was not that big a loss. Mainly since I shoot at night with the moon and thus I need to be able to stack, not leave the camera open for 1 hour, but instead take a long series of exposures and then combine them. You just get getter results this way. Here you can't use the P45+ as you end up with the dark frame each time, which ruins the sequence. Also in reality, if you are shooting say a 40min shot, it's pretty much one battery one shot as you still have to have enough battery to take the corresponding Dark frame. If your battery expired then the whole shot is gone.
As stated before the P45+ was a benchmark and a breakthrough. Phase was able to break new ground here. But after working with the newer Dalsa chips I am more than pleased to lose my 1 hour ability. Canon and Nikon fill that void very well.
Now with the reach of the D800, I have taken mine to 45 minutes just to see what I got, the 1 hour time of the P45+ is less important. Plus many are using the P645 Pentax at times of 1 hour or more and getting very good results. Not sure what Pentax is doing to allow longer times/less noise as I have never shot the P645, however I have worked with a few night shooters who have and their results were very impressive. From my understanding, the Pentax and P45+ ue the same Kodak chip.
Paul