This is a good question. Recently, one-piece bathing suits have really made their way back into the mainstream as a fashion statement. However, if you are asking this question, there are a number of factors you will want to consider when making your choice.
Comfort: When it comes to a bathing suit, you want to feel as comfortable as possible. This means that it may take some trial and error to figure out what kind of swimsuit (one or two pieces) feels best on you. Comfort means that you feel good in the bathing suit, so you feel confident in it and you won't need to re-adjust it all day long.
Use: If you are a very active person, you may want to consider a one-piece bathing suit that will keep you feeling firmly supported during an activity. However, there are a number of sporty two pieces on the market today, and they can be just as useful during activities.
Fashion: At the end of the day, you want to know that your swimsuit looks fantastic (at least to you). When it comes to choosing a swimsuit that looks good, you can easily consider either option (one or two-piece). It really depends on how you feel in the suit, and if you can do the activities that you want without worrying about losing your suit in a frisky wave. Just remember, if you feel great in a swimsuit, it will look great, because you like how it feels. So make the choice based on your personal preference and do so with confidence.