I would personally pick number 2, but go with which ever one you feel more comfortable in - after all that is what matters.
Here is the way I test a dress (other than liking the way it looks): are you comfortable sitting, walking, standing, and most importantly breathing in it? Can you get in and out of a car alright? Do you have shoes, jewelry, and a purse to match?
Also, having learned from past experiences, Spanx (slimming undergarment) are a must when you wear a dress!
Have fun at the prom!