Hey, Eric,
As you know, I am a happy AFi-II 10 user (and that is saying something). I was speaking with a friend who has a 12 (and a 10) and his initial impressions are that he felt the 12 was even more battery-hungry than the 10, and that the 12's 0.67 fps was slow, but not that big a deal for him. He praised the 4:3 aspect ratio of the 12 vs the 3:2 of the 10, and is a big fan of the 60MP square.
Since you're not on an Aptus-II now, let me reaffirm what Geoffrey said above, about the ergonomics--they're second-to-none. And you and I have shot together, so I can say that from having observed your shooting style (preference for the chimney finder, for example), I think you will love the tilting screen.
He wan't able to do IQ comparisons, as his initial back did have a defect. As I understand it though, the issue is being taken care of.
I guess since they share the same sensor then the IQ backs have nearly identical image quality?
This is a difficult question to answer, because so much IQ work is being done in software nowadays. As I'm sure you are aware, the way charge is handled as it is moved off the CCD and digitized (converted to voltage) is hugely important to final image quality. I think only Phase knows at this point what circuitry the Aptus-II 12 and the IQ180 share beyond the sensor itself. It would *seem* to make sense to only do the R&D once, but there are many factors not visible to us on the outside which could have led to separate implementations.
If you are interested in native file quality, it will probably be revealing to develop files from both backs in a non-native software (ie. something other than C1--DCRaw might be ideal to do an apples-to-apples comparison of file quality). That being said, you are a C1 user, so you will be getting the optimal raw file IQ available from the back. Once developed in C1,
almost everything I have come across so far seems to indicate that the resulting IQ of the 12 and the 180 is comparable.
In my own analysis, I've seen some evidence that the 180 may have superior resistance to purple fringing (sometimes due to blooming, but other causes exist). Nothing definitive yet, though.
As you know, I am taking a close look at the 12, myself. Happy to post more on the 12 as I get it...