I use both iPads regularly. I find myself taking the iPad mini with me where ever I go (both on trips and just in my work bag), while I tend to leave the regular iPad at home.
A retina iPad mini may tempt me to only use that device, though I do still like the larger screen when I’m sitting at home on the couch. My main issue with that device is the weight, and that’s supposedly being remedied this time around.
I have a Kindle Paperwhite as well, though I only tend to use that while reading before going to bed (or on a sunny beach).
I know I’m a device outlier — most folks will only get and use one of these devices. My guess is that a retina iPad mini will be the right call for most people. But we’ll see next week!