Depends on the custom where you live and your definiton of formal and semi-formal. How do the regular theater goers dress in your neck of the woods. I fear that around here (Atlanta), they'd likely look like they were going to a hootnanny or hog killin'.

In fact, "semi" means black tie, or tux, which would actually equate with a PC and black bow or straight tie.

"Formal" means white tie, i.e., tails with white bow tie and waist coat. That would equate with a doublet or even a PC with white bow and waist coat.

However, in the current environment of what is, IMHO, very slovenly custons of dress, you could probably wear a tee shirt and leather jacket and go unnoticed, at least as unnoticed as you can be wearing a kilt. Personally, I'd opt for the black tie look.

And the answer to the question about not owning either, the answer would be the same as the answer to every school boy going to a prom: rent.

Post photos of whatever you decide.