A few thoughts on the P45+,
1. IMO the only CCD back that can really get a 1 hour exposure that is worth anything, i.e. a normal full resolution print. I worked with the P45+ for 3 years and used the long exposure mode quite a bit. This is a iso 50 only. The files will be much cleaner than a IQ260, or 360, and most likely the 380. I have yet to see any comparisons between the 260 and 380, but own the 260 (not a smart upgrade for me) and it can barely go 15 minutes before noise becomes overpowering.
2. The P45+ needs 69 degree outdoor temps for the full 1 hour, any hotter and you will really start to pick up some noise. This is true also of the 260.
3. Early versions of the P45+, (mine was one) would not go to past around 25 minutes. Phase One needed to change out some parts on the back to allow this. You also need to be at the latest firmware to get the full 1 hour exposure. If you buying a used back, make sure the dealer or third party certifies it can reach 1 hour exposures clean.
4. The P45+ has no micro lenses, or if they are on the chip, they are not as big an issue as they are on the Dalsa chips. Thus the P45+ can take full movements from the Schneider 35XL and possibly the 28XL, full movements up to around 15mm of shift with no color cast. There will still be light fall off, but that's just part of of shifting. Net the P45+ is a great back for a tech camera in this regard.
5. The P45+ is only going to tether in firewire, so in the field, with a tech camera you will need to a full laptop to tether. The LCD on the P45+ is the same as the P30 so checking focus is a bit hard, at least for me it was.
6. The iso range on the P45+ is 50 to 800, the real range is around 50 to 200. In good light 400 can be quite good, but 800 will always be a real push for a full sized shot.
7. The P45+ is close to full frame 1:1 so a very small 10% crop sensor
8. The P45+ will not work with the new XF camera, now or in the future, per Phase One. So this will limit you if you are using a Mamiya mount back to a DF, AFDIII or DF+. With the XF shipping it's safe to assume there will be a lot of DF+ bodies on the market at a very good price point.
9. With Capture 1 Vr 8, you can get some excellent results from the P45+. I have been very pleased with some of my work that was taken from 2008 to 2011, all with P45+. The color was always great, but the shadow recovery has really really gotten very good with C1 Vr 8.
10. In summary, P45+ solid back, great chip, very durable construction. I feel it will give you more latitude in your photography than a P30+, especially if you are interested in the tech camera route.
Paul Caldwell