I recently faced the same decision, a loaded Mini vs mostly loaded iMac for a music production desktop. I bought the Mini 2.6 quad i7 with 256 GB SSD and will be installing a second SSD (256 GB Samsung 840 Pro) and 16 GB RAM this evening.
Reasons:
-I have a monitor, keyboard, and mouse already. While the iMac display would be very nice, I don't do graphics work and don't need it.
-Same goes for the discrete graphics card; I don't need it for music production, nor do I want the temptation to spend free time gaming.
-My audio interface uses Firewire; no need to buy a TB to Firewire adapter with the Mini or worry about any issues that might arise as a result.
-I would prefer to have two SSDs internally on SATA3 (one for system, one for audio data) than have a fusion drive for everything. Had I gone with the iMac, I would have wanted to buy an external SSD (USB 3 or TB) for audio data, which is yet another expense.
-Really the only thing tugging at me was the difference between the 2.6 i7 and the 3.4 i7... I rely heavily on software synthesizers and use some pretty processor-intensive plugins. I'm happy to report that the 2.6 GHz i7 is quite capable; it handles u-he Diva (a very demanding analog circuit modeled soft synth) fine at the highest quality setting with reasonable polyphony.
-Minis hold their value, and I can upgrade at any point if I feel I need more power.
It comes down to whether you need the display, and how much you're willing to spend. If money is no object and you think you'll need the power, then get that 3.4 quad i7... Otherwise, you'll be quite happy with the fully spec'd Mini and you can spend the money you save on something that will actually improve your sound.