I also forgot to add, wear comfortable clothes. Smart/casual does not mean a suit. If you wear a suit, to me, all it means is that you do not pay attention to detail on the invitation letter, you will not be as relaxed and so will not perform as well as you could and as well as they want you to; and you will also demonstrate that you have not visited an ATC environment before.
Whoa, hold on there!
Some of us wear suits at work....I certainly wouldn't think that someone coming to an interview wearing a suit obviously hadn't visited a unit. Although what you wear to an interview doesn't matter, as it says, I like to see a professional appearance. To me, wearing a suit would imply a high attention to detail.