I'm more for casual clothing than both. I mean, I'm from the suburb so usually when I go to a show let's say at night, I'll go down to the city early and spend the whole day there, whether it means going to softball, hanging out or whatever, and I really don't want to spend the whole day too dressed up. Also, the argument you paid so much, you should dress nice, I usually pay $25 for rush, etc. so it's not that much...and being such a broadway freak, I am an excellent audience member, being in jeans is not going to make me act at all inappropriate. I mean some people like even dressing a little nice, but I personally just like being comfortable, and I don't think there's anything wrong if I go in a T-shirt and jeans.
To the original poster, my advice would be to go with what you're comfortable with. If you have no problem dressing a little nice, or even like it, do that by all means. If you're a lot more comfortable more casual, just do it, don't worry about appropriateness, you certainly won't be the only one dressed casually.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli