I am upgrading from a 4th Gen iPod Touch. Remember that even though apple said that the specs/ processor are the same in bother the mini 2 and Air, there will be increases for the Air to justify the extra $100 (as it is not only for an extra 2 inches). There was leaks that the mini 2 has 1Gb of RAM but the iPad Air will have 2Gb (I find this likely).
The GPU the iPad Air is getting is different from the 5S as the 5S was as powerful as an iPad 4 GPU wise ; and the Air is 2X that so it will most likely be the G6630 for the iPad air.
When apple revealed the 5S, the said 2X increase in graphics but in terms of Gflops, it was a 2.67X jump. So if the ipad Air's 2X jump is also this, is will perform around 205Gflops. I see the mini not being overclocked but the Air will be.
iPad Air
A7 1.4Ghz
2Gb RAM
G6630 GPU
iPad mini with retina ($100 cheaper)
Smaller screen
A7 1.3 Ghz or 1.4Ghz
1Gb of RAM
G6430 over clocked GPU.
Apple have done this a few times where the make it look like 2 devices are speced the same (iPhone 4 and iPod touch 4G for example), but is turned out the iPhone 4 had twice as much RAM and a much better screen. The same will happen here as $100 is too much for just 2 inches. I will buy the Air with the hope of increased specs.