formal night planning

I'm trying to decide how formal we should go for the two formal nights on our trip.

I've got two really great gowns, a spaghetti-strap black gown with beading on the bodice and a matching jacket, and a blue gown, also with spaghetti straps and a matching jacket, with beading all over the gown and the jacket.  I wore the black one for my older daughter's bat mitzvah and the blue one for younger daughter's bat mitzvah.

Significant other owns a tux but really doesn't want to pack it.

This could get interesting.