The only image quality differences are that the IQ has:
- availability of high ISO using sensor+ (up to ISO3200)
- the availability of ISO35 and 2 minute exposures
- different color renderings
Arguably you could call both of these "features".
Focus mask, auto horizon correction, two-way on screen digital level, fast bootup time, faster tethering, better raw file compression (smaller files without lose of quality), extremely durable chassis, no moving parts, availability of a 5 year warranty, and other details separate the two, as well as the more obvious (above mentioned) brighter, more detailed, faster LCD interface.
By the way, the Aptus II 12 has some unique advantages of it's own. Unlike the IQ the Aptus II series:
- allows the user to enter custom metadata like the lens length/aperture and notes
- is available in a Hy6/AFi mount
- is available in a V mount which allows internal rotation of the sensor from vertical to horizontal
- can apply a stored LCC in-camera
If absolutely all you care about is the final image quality then the Aptus II 12 is a steal when compared to the IQ180. I would strongly argue an IQ180 allows you to more quickly, more confidently, and more easily narrow in on the exact shot you want and confirm that all attributes of the image are as you want them (focus, composition, exposure, detail etc), which can make shooting more enjoyable and a more confident experience. That said, many many photographers shot (and still shoot) film with nothing more than a polaroid (often not even that), so these all of the features of both digital backs are very nice luxuries not dire necessitates.