Ionic is the best way to go. Traditional dryers use old school heating methods such as plastics and metals that literally boil the water off your hair. Ionic dryers produce negatively charged ions and trap moisture inside the cuticle having it lay flat and eliminating frizz.
Settings are also something to keep in mind. I prefer high,medium,and low speeds depending on the style and texture of your hair. Make sure that your dryer contains a high,medium, low heat and a cool button.
Be weary of heavy dryers! The lighter the dryer the less tired your arms become and you can spend more time enjoying styling your hair.
Attachments are something you should consider. I always have a hair concentrator which is the key to silky smooth styles. If you are curly haired gal, a diffuser is a must!
Just remember to keep it simple and checkout reviews online. Getting your new dryer should be fun and exciting.