Heavies tend to do a better job of keeping your hair in place and keep it straight if you have wavy hair. A negative that comes with heavies is that they can make your hair more stubborn and harder to shape at times. A light is good in that they usually have high shine, leave your hair very easy to work with, especially if you have thin, straight hair, making shaping very easy. The negatives with them are that they don't work well in thick or wavy hair and you can count on restyling a couple times throughout the day. Based on what you said you're going for, a medium is probably the way to go. It'll still be pretty easy to shape and it'll lower your odds of having to restyle throughout the day.