Being a hosiery junkie, I do love both, so which one I'd choose would be based on how short my dress would be. If the dress was somewhere around the knee or longer, I'd go with stockings and garters. If it was a higher hemline, then I'd opt for pantyhose, so as not to freak people out. It is for a formal occasion, after all.
I am a guy but I think that for any formal/professional occasion a girl should always have her legs covered. Michelle gave a great answer, it really does depend on the dress length. Though I think stockings and garters with a short skirt/dress would be sexy as hell.