Where are you getting the idea the stock mini will outperform the iMac?
An upgraded Mac Mini has,
CPU - 2.6GHz i7
This is a mobile class CPU that has a lower clock speed. It does have hyper-threading (virtual cores) and as far as I know Photoshop doesn't tend to take advantage of this and watching video certainly won't, not sure about Aperture.
While it is one generation ahead of the iMac's CPU, the iMacs has a faster clock speed (3.1GHz) and is a proper desktop class CPU.
GPU - Integrated HD Graphics 4000
This is simply no competition for the iMac's 6970m.
RAM - Well 32GB vs 16GB is obviously better, of course that's assuming you will actually use all 16GB and more.
HDD/SSD - You didn't say what the iMac has but HDD/SSDs can be upgraded on the iMac anyway so I guess it's a moot point.
On paper, the only way I could see the Mac Mini outperforming the iMac is in the few instances you might be able to actually take advantage of the hyper-threading the i7 offers. Otherwise the iMac wins with more RAM, better GPU, better CPU and room for larger capacity HDDs. (Also note: the iMac's DVD drive can be replaced with an additional HDD or SSD for a total of 3 individual drives.)
I would ask yourself these questions and base your decision on your personal answers.
Do you need the hyper-threading of the Mini's i7?
Do you need the dedicated 6970m graphics card of the iMac?
Do you (or will you in the near future) need more than 16GB of RAM.
Do you want an all-in-one iMac or a bare bones Mini and provide your own monitor, speakers, etc?
Which one is cheaper? By how much? And is it worthwhile saving that money?
