There is a difference in performance. But a 2 year old Mac Mini has more performance than a 6 year old iMac. I have gone 6 years without an upgrade to my iMac, but would have upgraded a Mac Mini a couple of times already.
The depreciation on a Mac Mini is so low, and the relative ease of sale, means that regular replacements is cheaper and less hassle.
And I use it as a "tool" for my business.
Personally, I don't believe that a 2-year old Mac Mini has more performance than a 6-year old Mac Mini (using same RAM, similar graphics and equivalent processor), unless you have upgraded your Mac Mini 2-3 times by your own (apart from RAM)....A new iMac needs guts to be upgraded (without obvious damage)!
Of course, a Mac Mini is more cost-effective...and my correction, for small business, a Mac Mini may be a useful "tool", too!