The Mac Mini has less processing power than an iMac. So if you're a photographer who's going to be doing some digital editing you might want the extra power.
I would have to agree with this. The accelerated GPU in the iMac will be able to offload a lot of the graphics work from the main CPU (if you're using applications like Aperture/Lightroom or Photoshop). With the Mini, the processing will be primarily handled by the main CPU, so operations will almost certainly take longer.
Also, the hard drive in the iMac is going to be faster (7200RPM vs 5400) and so any disc intensive operations (including any page swapping) will be notedly faster on the iMac. This can be solved by replacing the Mini's internal drive (if you're feeling industrious) with a faster 2.5" SATA drive (I believe 7200rpm drives are now available up to 200gb in size). I had thought that booting from an external firewire drive would increase performance, but
this page shows that it's actually slower (FWIW, that link goes to a page that describes tethering an external SATA drive to the Mini. This nearly doubles the performance of read/write operations, but leaves a bit of a mess of cables).
By no means am I suggesting that the performance of the mini will suck, nor will it likely be unusable. Just simply, the iMac will be a fair bit faster.