If I had to choose one (and I probably will after Christmas), it will be the mini w/ retina, despite its lame steep price.
I currently own the iPad 4 and Nexus 7 and I like the iPad for some things and the Nexus for others. I feel that ~8" is the perfect tablet size; 10" is nice but a bit cumbersome for book reading and extended usage (the weight reduction of the iPad Air will probably help in this aspect, though). 7" has great in-hand feel but is a bit small for surfing the web. The iPad mini's size is perfect IMO. Plus Apple's tablet ecosystem is still better.
This holiday season, if I can get my hands on a Retina mini, I will probably consolidate my tablet usage to the iPad mini w/ retina. I hate paying the Apple tax, but until Google releases an 8" Nexus tablet with a nice screen, I couldn't even consider consolidating on an Android tablet at the moment.