I have the dual-core i5 MINI at home and the 27" quad-core i5 iMac at work...
IMO, different strokes for different folks. The pros and cons are as follows:
MINI PROs: Small form factor means the tiny workspace I have at home won't be taken up with a giant computer/monitor or combo, easy to upgrade RAM and hard drive, flexibility in choosing what monitor I want.
MINI CONs: Runs HOT due to tight enclosure, USB ports all used up with keyboard, external DVD drive, USB drive, USB Printer, FireWire RAID array, computer speakers...etc, creates far more clutter than it solves. Oh did I mention it runs HOT? Encoding video in Handbrake can easily see 100+ degree CPU temp very quickly. Can only accommodate 2 ram sticks, so currently maxed out on 8 GB of ram
iMac PROs: Giant, gorgeous 27" monitor, exceptionally high resolution screen means I can fit several documents on the same screen, and have room to spare for all the application pallet. 16 GB of ram kicks butt. Quad core kicks butt. Comes with Wireless keyboard. Runs exceptionally cool (hasn't seen more than 50º C yet)
iMac CONs: Comes with wireless keyboard, so no numeric pad. WTF. Much harder to upgrade hard drive. Stupid GLOSSY screen makes for terrible glares, but good for spotting boss coming up behind your back. DVD slot and SD card slot on the left hand side which is where I'm used to putting my 2ndary monitor.
The Mac MINI has a 256 GB solid state drive, so boot up, application loading, and operation in general seems much snappier, but since it's a third party SSD, occasionally I get these weird hang-up when copying multiple files to multiple locations over several different busses at the same time. Since the iMac only has one single platter drive, Finder previews, opening files and apps...all that seems to drag compared to the MINI, and I'm too lazy to want to try and upgrade the work "workstation's" hard drive, even though it'll take another SSD easily.
If I can put the SSD in the iMac (and swap for a non-glossy screen), or the processor in the iMac in the MINI without it melting, I'd be a happy man. Right now, the way it's configured, I enjoy working on my InDesign files and doing Illustration more on the MINI, and love encoding Final Cut videos on the iMac because the 2X memory and 2X core. But if you were to put a gun to my head and choose? I'd take the iMac and get a SSD.