Here are a few more thoughts:
1. If you purchase a P45+, it will be used. Make sure you purchase from either a authorized Phase One dealer or if you choose to go outside the dealer channel make sure to test the back before you buy it. Many early P45+ backs would not get all the way to 1 hour exposures without a controller card change. I owned one of those. DT helped me through the fix. A authorized dealer can certify the back, and give you a good solution.
2. The 1 hour exposures outdoors are limited to 69 degree F. If you go 1 hour with outside temps higher, the quality will fall off very fast. In my state, it's only 69 degrees at night about 6 months of the year. You can still shoot shorter, say 30 minutes or less, but the 1 hour will not look good, it will pick up a strong red tint. If you are working in a high humidity environment, this 69 degrees may drop even lower.
3. The 45+ will require a dark frame that is as long as the previous exposure, i.e. 1 hour shot, 1 hour dark frame. You must have continuous power during this or the frame will be lost. On a older Phase One battery, you will get about (2) 1 hour shots, that is the first and dark frame. You might get (2) 30 minute after that. You can run the P45+ from a Canon video camera battery and get much longer times. Do a search on this on this site as there are man pictures of it.
4. The P45+ will work great with the lesser priced Schneiders, and older Rodenstocks along with the newer HR HW Rodenstock and Digitar Schneiders. You will be able to shift to a full 20mm on say a 43mm Schneider with almost no red/magenta cast. You will still see light fall off, but the LCC totally fix this. The P45+ is a great shifting back.
5. I found the LCD on the P45+ is hard to work with period and if you add a tech camera then it's very frustrating. , I know many out there work with it but outdoors it's almost impossible to read it and it takes very long to refresh i.e. if you try to zoom into a 100% view. Also the 100% view can cause more trouble than good as many times it fooled me into thinking I had a good focus, but in reality I was not there.
6. 645D has a very modern LCD, allows a 100% view that can tell you a lot more data.
7. The P45+ with the latest firmware will shoot a very nice iso400 and will give you a useable iso800. The 645D is rated to 1600, but I have not seen anything from that high, I have seen the results at iso800 and it's very useable.
Both are very good cameras see if you can get a demo of both.