Got mine today. Very nice little device.
Compared to my Nexus 7 (2013), the first observations are:
- Retina Mini looks nicer and has the higher quality feel, N7 feels nicer in the hand, especiall when you hold it in one hand
- Retina Mini is better for reading fixed format PDFs and browsing (due to 4:3 screen format)
- N7 has better screen quality (retina Mini's screen is not bad at all and has the better format, but the N7's colors are more vibrant in direct comparison)
- N7 is better for reading ebooks that don't have a fixed format like Kindle, epub (more comfortable to hold, screen is somewhat clearer to read)
- Retina Mini is FAST! N7 is no slouch, but the way websites pop up on the Minis just like bam and they're there is something I'm not used to on mobile devices.
Overall, I'd say the retina Mini is the better and more versatile tablet, and if I had to buy only one tablet, It would be the Mini. N7 still is a great alternative with an amazing screen but for me the 4:3 screen format and the (IMO) better app quality are the difference makers.
Also, I was kinda unsure between the Air and the Mini, but I'm glad that I got the mini because (for me) it's still big enough to read everything on the smaller screen while being much more portable.