Very good question. I would say to weigh how formal the meeting is; do you have an appointment with the owner, HR department, fitness director? Is it a meeting or are you dropping off your resume? If it is a sit down meeting I would opt for standard (non fitness) business dress. I do not believe you can go wrong dressing this way. However, I have had interviews in between clients, so sometimes there is no choice. If it is more causal I believe very professional workout wear is acceptable. This means no slogans, no tanks, no shorts, nothing tight or revealing, etc, keep it neat and clean.