Well, unless they've vastly improved Antigen, I wouldn't touch it with a bargepole. It might be powerful, but god it slows things down.
For a small business, I've had good results with Avast! Certainly the price is much cheaper than elsewhere, and the scanning appears to be robust.
Just to throw another idea into the mix, have you looked at external AV/antispam providers, like Postini or MessageLabs? They can provide pretty cheap solutions for SMBs - it might be worth getting some pricing there. You certainly get savings in management costs