I own both an Air and Mini right now (1st gen). I haven't used the mini at all since buying the Air, I am just waiting on selling it at the moment. Of course the retina Mini is faster and better display, but for the most part I prefer the bigger screen. The better combo is Air + Kindle Paperwhite - Air for email, web, game and other apps with the Paperwhite for reading books. The advantage over Air and Mini is that you won't be constantly having to charge (The kindle lasts for around a month.)
But of course that would be assuming you want a Mini mainly for reading.
I've been thinking about this exact scenario for myself, though using the rMini as the eReader as opposed to the Kindle.
The mini (I've had a couple of Nexus 7 tablets around) seems to fit a particular need for me, mostly around eReading and "carry everywhere" type portability. I do wonder about it being too small for regular web-browsing even around the house.
I currently have a Surface Pro 2, which quite honestly has been an excellent tablet and ok computer, but at 2 lbs is a bit much to lay in bed and read from. It's excellent for watching videos and actually being productive. But I really mostly use it for watching videos, and couch-surfing. It's occasionally useful as my "backup" PC at the office (use a 13" MBA with a monitor the rest of the time).
I'm thinking of swapping the SP2 for an iPad Air as my video watching / couch surfing device. I can't justify the iPad Air at all....but every time I use one, it doesn't change my desire to want one.
Funds aren't the issue, but I do try to minimize the number of devices I cart around. MBA+SP2+rMini or do I go MBA+iPad Air+rMini (1 lb lighter)?
I suppose if I was truly smart I could go SP2+rMini and leave the MBA at home, and use Windows 8.1 as my primary OS.
Realizing that this is MacRumors, so an inherent bias is built-in, but would you go SP2+retina Mini, or MBA+iPad Air+rMini, or just MBA+rMini (or iPad Air)?
If cost wasn't an issue, would you have both the iPad Air AND a retina Mini? Or is the idea of having two tablets (regardless of cost) a silly idea?
Patrick