As mentioned - either will do - it really comes down to cost and what you can afford. If you already have a Dell system - printer etc - then you are the target market for the Mini. Just move your important data files to either a USB memory stick or burn them onto a CDROM. Unplug your PC - plug in the Mini - you're good to go.
Things to consider on the switch - niether the Mini or the iMac support the old style parallel printers - or - old PS/2 keyboards / mice. Also with either machine 512MB memory is the minimum you need. and with either system you'll want a USB2 4 port hub ($40) for added flexibility.
I'd look at it this way-
If $$ are an issue - get the Mini so you can at least make the move to OS X and Apple ------ you'll never regret that -- in a couple of years or so you can upgrade
If $$ are not an issue - go for the iMac
Something else to consider - as you start to build a "digital life" (pictures / music on your mac) you should invest in a 60GB USB2 (or firewire) external drive to back your stuff up --- bad things can happen on any computer
GOOD LUCK !!!!!!!!!!