Most people will not be able to see the pixels on either display, so at that point it'd be all about size preference. Some people will want a smaller package to easily carry around, while others will want more area to work and view shtuff. I'll have to check out a retina mini at the store to see if its worth the size drop. I lovvveee my big iPad.
I have no doubt that some people will complain that the full-size Retina iPad looks like crap compared to the Retina Mini, that it's obsolete and unusable, and that Apple is screwing over its customers by selling old Retina technology at inflated prices.
Seriously, I'll be curious to see what the Retina Mini is like in person. Last fall, I compared the iPad 4 with the iPad Mini, and while I loved the Mini's light weight, the iPad 4's screen blew me away, so I bought the 4. I see no need to upgrade this year, but if I like the Retina Mini, that might be the next iPad I get. Other factors to consider will be weight (will the Retina Mini have a bigger, heavier battery than the current version, thereby limiting some of its appeal?), the processor (will Apple differentiate the full-size iPad by continuing to give it a faster processor?), and storage (will 128 GB – which is important to some people – only be an option for the full-size iPad?).
What difference the screen size makes will be subjective, and I can't yet tell what I'll think until I see a Retina Mini in person. While I love my iPhone 5, I don't use it for nearly as many tasks as I did before the iPad came out. The screen is just too small for me for Web browsing and serious typing. I only use Safari when I'm out and need to look up information. At home, I use my iPad. And while photos look great on my iPhone, they look even better on my iPad, because the image is bigger yet still "Retina" sharp. If the reduced size of the Retina Mini's screen doesn't subjectively diminish its impact and usability for me, then I'll probably go Mini next time.