Wear the suit. I work as a headhunter and that is what I advise every single applicant that interviews with one of my clients. It doesn't matter what the company's standard of dress is. It doesn't matter how the interviewer is dressed when they sit down across the desk from you. A first interview is entirely about the employer making an initial assesment of you, and like it or not, whe we interview people we are predisposed to think better of them when they are dressed professionally. This would be especially true in a sales role, because a salesperson is really selling themselves as much as their product or service. First impressions are everything.