Anandtech have written a fabulous review comparison of the iPad Air and iPad Mini Retina.
“When I concluded our iPad Air review I assumed the iPad mini with Retina Display was a true no-compromise, smaller alternative to the iPad Air. In many senses that’s true. Wireless connectivity is identical between the models, battery life is pretty much the same as well. Peak performance is close and you no longer have to accept a lower resolution display. Last year’s iPad mini was easy to recommend, and this year’s is even easier. To my surprise however, the iPad Air continues to hold some advantages that may resonate well with some users.
The biggest in my eyes is the iPad Air’s wider gamut display with full sRGB coverage. The mini’s Retina Display is good, the Air’s is just better. There’s also more thermal headroom on the iPad Air, which can come in handy if you’re doing compute intensive work on it. If neither of those things matters to you, then the decision becomes one of usage model and portability. I believe the iPad Air does a better job of approximating a primary computing device, particularly in its ability to give you a reasonable sized virtual keyboard to work on. The iPad mini on the other hand is substantially more portable. Although the iPad Air is light enough to come along with me more than any prior iPad, the mini’s form factor makes it even more likely that’ll I’ll bring it with me (the best tablet is the one you have with you?).”
If you read one review of the iPad Air and iPad Mini, this is the one to read.