Girly or casual. Girls trying to be classy don't do it for me, they make me eyeroll. The girly look makes me think she's fun and just likes to look pretty. Classy makes me think you're trying to get everyone believe your all sophisticated and super educated. No matter how sophisticated and educated someone may be, when they try to show it off in their look that's just off-putting. Give me a super smart girl in chucks and jeans any day!
I'd choose casual... its more comfortable, which means she will be comfortable too... win win. but on a much more deeper level... doesn't matter what she wears, all that matters is how easy it is to take off lol XD