I've thought about this for a while now, and I think -- for me -- the 2014 mini is better than the 2012.
Nothing I will be doing on the mini will really benefit from a quad-core architecture, but will benefit from faster single-core performance, which the upper-end 2014 machines do have. For me, it'll be an office machine, not a rendering/video encoding machine, so the benefit of four cores is low. I suspect I'll settle on the 3.0GHz i7, which is faster in single-core threads than the 2012 i7 2.3's or 2.6's.
I'll be maxing out the RAM anyway upon purchase, so upgradability after the time of purchase is moot. Yes, cheaper to do it myself but over the expected life of the machine, the difference is at most $40 per year.
SSD upgradability is possibly a true concern, but a 256GB internal SSD I believe will be sufficient for the anticipated life of the machine (for me, about 3 years). As long as it's enough to hold the OS and apps, data can be on an external device. And TB2 provides ample speed for a fast external if it proves necessary in time.
Two mDP displays up to 2560x1440 thanks to two TB/mDP ports - something the 2012 can't do (one TB/mDP port and one HDMI limited to 1920x1200).
I totally agree that for many things the '14 is inferior to the '12. But for me, I believe it'll actually be better.