I've got another X-mas party to attend!

This one is for M's work. She has promised to introduce me as her wife/partner to her coworkers. Some of them already know about me, some have actually seen me, but the rest know of me only as her roommate.
Anyway, I feel like a debutante. But what's a girl to wear? Its supposed to be dressy, so which dress do I wear? Black or cream? Sleeves or no sleeves? Long or short? Right now I'm leaning toward my longer, flapper style dress, with very high heels, since her supervisors will be there... but I'm not sure. Any suggestions dear readers( and by readers I mean M and E, my only two readers)?