Ok, I see why you're feeling unsettled. It would actually be easier if you didn't have the casual option.

In that case, I'd go with slacks and a sweater or nice blouse with a cardigan over top. Kind of "business casual".

I'm sure they'll get a real mixture, but remember: first impressions ARE important, no matter what they say to the contrary. I wouldn't go the jeans route...doing just that little bit extra to present yourself professionally will go a long way to sending a message about how serious you are about this job.

Good luck, sweetie!