Prom Hair.. Advice needed :)?

Hi my prom is in July and I am still trying to decide what hairstyle to have. I am a big boned girl and big chested. I am a size 16 and 5"9. My prom dress is black and it's a halterneck, I am stuck on what style would suit me best according to my dress and my face.. which is oval-ish with high cheek bones, I also have longish brown hair. I don't have a clue whether to opt for up-do which will show off my halterneck or go for a big down hairstyle that will balance me out. :S Any advice would be appreciated, thanks :) xx