Why keep the P45+? Would it be sitting on a shelf collecting dust? Or do you do a lot of long exposure work? The P45+ seems to be the king of long exposures so that would warrant hanging onto it. I can't think of another compelling reason.
I've played with both the Aptus II 12 and IQ180. Not long enough to comment on any difference in image quality but the IQ180's interface is a lot faster. Would that help you to take better photos, or is it just 'nice to have'? Would the price difference be better spent on other accessories? If you shoot tethered, you don't need a nice or fast GUI on the back. Funnily enough, I think wedding photographers would really appreciate the IQ backs the most. They move around quickly, untethered. They do get a few chances to take each shot before quickly moving on. The IQ180's fast previewing would be hugely helpful there, to check focus, closed eyes, etc on group shots before moving on to the next group. Having said that, I doubt it makes any commercial sense to shoot a wedding on such a back!