I think the first question would be, what are you planning to do with a mac...
it's my understanding, the mac mini is basicly for people who are just surfing the net and running MS Office... but based on the tech spec's for the current mini, it seems to have evolved. the mini has more usb ports, which, may or may not be a plus, depending on what you want to hook up... also i don't think the mini comes with a keyboard, which may or may not be an issue.
the ACD is a waste of money (IMO) since you can get the same sized display/monitor by another manufacture for a fraction of the price at places like costco, best buy or other computer stores.
at $1400, you could set your self up with a Macbook that comparable, if not more powerful and a large monitor.
i'd say go with the imac... but that's an opinion. I'm sure others might have a different opinion