So, like I said, the Mac Mini is actually overdue for an update based on past trends. The rumor mill has been swirling strongly for the past 3 months that an update was forthcoming, and just last week a UK retailer's site leaked a page for the updated mini, with a sales date towards the end of the month, but without updated images or specs. This still very much amounts to a rumor, but its the kind of thing that precedes an actual product update more often than not. The iMac has had it's scheduled update and so you're right to say that its specs are quite a bit ahead right now. The Mini's update is likely to bring a Haswell CPU, Iris graphics, flash-based storage, and 8GB ram as standard, so it will be much more inline with the iMac, which has already received those upgrades.
Personally, I don't much like the all-in-one designs like the iMac, but its a personal preference and to be honest, no current mac is all that much more upgrade friendly than the others anyway. I mostly just dislike that I may end up with a broken screen tied to a working computer, or vice-versa. YMMV.
Anyhow, if you don't need to get one today (and it sounds like you don't), keep an ear to the ground about the Mac Mini refresh, and then compare it with the iMac before you settle on one or the other, and get the best value-package you can.
And keep in mind that for audio work, what you mostly need is a good amount of RAM (16GB+), fast storage (local flash), and more speed/cores (audio work can be computed in parallel pretty well). Bulk storage isn't so much an issue, and you probably want to farm it out to an external RAID array anyhow.