Is there a relevant difference? The quad-core
mobile i7 processor in the Mac Mini server has Hyperthreading and Turboboost, and MACWORLD found that a BTO Mac Mini (using a dual-core i7 and not even the quad-core in the Mac Mini server) beat an i7 iMac.
Customized Core i7 Mac mini offers iMac-like performance | Macworld Intel® Core™ i7-2635QM Processor (6M Cache, up to 2.90 GHz))
I'm not an expert on the numerous variants of Intel processors so I could be off but I really doubt there's an appreciable speed difference between a Mac mini server and an iMac. (Of course with the iMac you get a monitor, Superdrive, videocam, keyboard and mouse [or trackpad.])