Fraternity Formal Attire?

One of my best friends is a new brother in Sigma Phi Epsilon, and he asked me to go with him to the fraternity's formal at the end of spring semester. But I'm not really sure what to wear? It is black tie, and I believe he's going to wear a tux, but I'm not sure if I should go all-out prom style or maybe a fancier short dress. I'd prefer a shorter dress, but I just want to make sure it's appropriate. Help!! Thanks!