If you get the quad-core i7 upgrade for the Mini, according to Geekbench, it looks like the CPU might be pretty comparable to a new base model 27" iMac. It depends on what you're doing and how much advantage your software gets from hyperthreading vs. having fewer (but faster) threads.
The Mini has the standard Intel integrated graphics, whereas the iMac has dedicated Nvidia graphics. This only matters if you play hard-core 3-D video games or you use one of the few software packages that take advantage of OpenCL (i.e., parallel processing using the GPU). I think 99% of users won't care for s*** about what kind of GPU their computer has, although probably 75% of users think, incorrectly, that they "need" a faster GPU.
There's more to it than that. I for example bought a refurb 3.4GHz imac this year. The Applecare that I purchased off ebay for $100 will add more than $100 to the price I get when I resell it to buy the Haswell iMac in 2014 because it will still have 6-12 months warrantee. So it's horses for courses regarding whether it's worth it or not.
Plus there are also intangibles that are harder to value. For example how do you put a value on 'peace of mind'?
