I briefly considered the mini, but there would still be two pieces, I'd need to get a new monitor as the 30 is still large, etc..
If I was going to go that route, I'd probably use a MBPr. It would get me about the same speed as the iMac, but with a max ram of 16Gb, and when I needed to shoot without monitor, edit on plane, etc, all my files would already be there.
It's a good idea though! If I already had a smaller monitor, I'd probably go that way.