I just went through the same decision for photography and video. By the time I got done with a BTO 2.6, a 480 gb ssd, a second internal hdd and an Asus PB278Q display, I was less than $500 from the highest spec iMac available with the 3tb Fusion. And I'm left with a modded new Mac that Apple can easily claim was damaged during the install.
I love the mini's, both I and my son have one, as well as an iMac. But I found trying to compete with an iMac spec left me with a lot less CPU, less gpu (which many graphics and video apps can use), SATA instead of PCIe for the ssd (SATA is already max'd out for ssd's), half the internal storage, one less T-Bolt port, more wiring, and potentially dicking around with display settings and cables.
Something to consider if you expect to push the mini. I have a friend with a 2.6 mini and doing post processing, the fans kick in a lot more often than he or I would expect. Our older 2.53 ghz mini's certainly kick in at the mere thought of offloading some video rendering over to them. Which means less than 2.6 ghz during the times you need it. Our 2.8 ghz iMac can hang in there a lot longer before the fans kick in. Not to mention a small fan directed at the large aluminum back panel cools it down very effectively. Not possible with a mini.