With apologies to the previous poster...
The iMac. Faster, quiet, portable (one box, no wires), elegant, not chained to a monitor (it is the monitor), etc., etc., 🙂
I have to agree with Jackintosh---excluding your first point, this argument makes no sense.
Faster? Certainly and if this is most important for whatever uses you have in mind, could be deciding factor, however...
Quiet? Can't comment on iMac but I have a Mini and it's very quiet, even with an upgraded 7200 RPM hard drive. I don't think I could tell if another computer was quieter.
Portable? Yeah, the iMac is portable--I see people unloading theirs from their suitcases for one-on-one sessions at the local Apple Store. The mini fits in a backpack. Oh wait, that's right, having to carry that ONE extra cable to connect the mini to a monitor may just be too much.
Elegant? This is purely subjective and I'd state the mini is beautifully designed but to each his own.
Not chained to a monitor? Simply incorrect and in fact, exactly the opposite but since this has been addressed by others, enough said.
My $0.02---go with the mini, especially if you have dual monitors. Tons more flexibility, great small footprint, as nutmac said, makes a great HTPC.