First of all, let me caution you about moving your business to a new platform, especially if you are planning to maintain your current key applications. Macs aren't a silver bullet, they aren't a panacea, they're a tool. And if the tool doesn't do what you need it to do, it's not the right tool. I've said before, a cheap screwdriver will drive a screw better than the most expensive, most well built, and most highly regarded hammer on the market.
As far as cost versus benefit of an iMac vs. MacMini. You're going to have more power, larger hard drive, better video card and more up to date hardware on the iMac. In addition, if this is going to be a computer used "front of house," i.e., where the customers will be, as seen on television and movies, iMacs are eye candy - people are just naturally drawn to them. If this is going to be an office system, visual aesthetics don't matter as much.