According to WikiHow here are some steps in choosing the perfect prom dress for you! The Mahalo link also has some great tips. Have fun and good luck. I would go for the 20s flapper!

1. Begin your search for your prom dress early. You may not have a date yet, but you will want to start looking at fashion magazines and stores as early as three to four months before your prom.
2. Make sure you don't pick one that everyone else will have, you should be different, just be yourself.
3. Plan your budget for your prom dress and begin saving as soon as possible. Be sure to allow extra cash for small accessories like hair clips, pantyhose, and make-up.
4. Watch some celebrity red carpet events and take notes on your favorite styles. You may just be able to find a similar prom dress in your local boutique.
5. Try on dresses in a variety of different styles and cuts to find the one that works best for your body. If you have a slim figure, you may look best in a sheath-style dress that shows off your silhouette. If you are curvy, you may want to consider an A-line dress that will highlight your waist and minimize your hips and thighs. If you're short and petite, dress shopping may be a nightmare; cocktail dresses become full gowns and party dresses become cocktail dresses. If you're looking for a longer dress, look for a short dress that is long on you. It will fit better and will fit you everywhere else without being too long.
6. Once you have narrowed your choices to one particular prom dress style, try on that style in a variety of different colors and finishes. Look for a glamorous shade that brings out color in your face. When choosing a fabric finish, keep in mind that shinier finishes will tend to show off figure flaws while matte finishes will obscure and diminish undesirable features.
7. Start shopping for shoes and accessories, this should be done in advance of 2 or more months for the Prom or Formal dance. This way, if you have chosen shoes and bag you can try it all on together at the Dress Shop, if you change your mind, you can at least return the bag and shoes if time allows, and you will only have to pay the lay away fee rather than getting stuck with an expensive dress no one else would necessarily want.
8. Plan to choose and purchase your prom dress at least four to six weeks before your prom. Most formal gowns (including prom dresses) require alterations for a perfect fit, and you will want to allow enough time to complete these alterations.
9. Make sure that you don't purchase your dress to far in advance, as before big events you can easily loose and gain weight, from stress or anything else, e.g. periods, boyfriend troubles, stress, season (winter versus Spring).
10. A couple of weeks before your prom, try on your dress with your shoes, jewelry, make-up, and hairstyle to be sure you like the finished look. Walk and move around in your prom dress a little to give it a comfort check.
11. On the night of your prom, put your dress on before applying make-up or styling your hair. This will not prevent any make-up smudges or product stains on your so called "fabulous" prom dress

It depends on what looks good on you. Here is a trick. Coerce a male relative (not your Dad or Uncle) to go with you when you try them on. If he is anywhere close to your age, he will tell you which one makes you look better (in other words, which one makes you look se...uh, more attractive).