I don't know the gap, between the new P40+ and the Credo 40, but I would strongly weigh the credo over the P40+ mainly due to the screen and if you are tethered shooter the USB3.
The screen on the older Phase Backs, like the P45+ and P40+ basically were good for general info, i.e histogram, exposure, but not good for determining good focus. I use the P45+ for almost 3 years on several different Mamiya 645 cameras, then ended up with the DF. With the older Mamiya wides, and even the mid telephotos, you can be fooled by the AF drive and miss by just enough to be out of critical focus. For me, attempting to manually focus with the optical finder on the DF is next to impossible, even with the Mamiya magnifier. In the field I found that I often times needed to use manual focus and the only real way to tell was viewing the LCD on the P45+.
This is a cumbersome process to being with, requires several different combination of buttons to get to 100% and then to move around. However the screen's feedback at 100% tended to fool me and often what I thought was in focus was not.
The IQ, Credo screen really has no comparison on the market currently from what I have seen. The feedback you get is very impressive. The Credo doesn't offer focus mask, but it still have the same clear sharp resolution similar to an iphone screen. Viewing images on this screen at 100%, you know instantly if you are are in focus or not and it's worth more than a P40+, but as I don't know the current gap, between the two I don't know if the screen alone is enough. In my work I am 100% dependent on the IQ LCD as I don't shoot tethered. If you are in a studio/tethered, then I realize this may not be anywhere as big a consideration.