not sure
It depends on weight of the iPad and screen resolution. I loved the Mini but it was murder on my eyes and the regular sized one was too heavy for me. So if the regular iPad is lighter I'd probably pick that even if the mini goes Retina.
And to speculate if Twelve South tweaks their iPad hoverbar to fit the thinner regular sized iPad and the 2012 21.5" iMacs. I've discovered the iPads are finally coming around in regard to content creation and to be able to use one as an extra screen on the iMac would be neat - the standard two monitor set up would be overkill for me.