You'll be a hottie in either dress, I'm also built like the two models so some insight from that perspective.
Green dress very pretty, very low cut for a gal with small boobies (I'm an A/B cup like barely B and thin too). I would and I'm sure you will too, use double backed dress tape to keep the girls from escaping! I wore something similar to my guys prom (he's a year ahead) even with tape there were a few dances that I had to be really careful not to be part of a side show!!
Blue dress is essentially the dress I wore to my prom, mine was crimson with a little gold sprinkled around to make it interesting, although I can't see the back, my dress is bare in the back cut just about 5 - 6 inches above my waist. Although I had to use a couple of strips of tape along the middle of the back to keep things in place. I had no issues with the second dress, got more compliments on it too! My guy loved it, it felt nice with his hand on my bare back when we slow danced. = ] The best part, since my parents would only pay for one prom dress, was that it was cheaper than the first by $15 - $20. I was able to dance and wiggle around him with no worries all night!
Either dress is going to look stunning on you I'm sure, just make sure you will be both comfortable and be able to move in it, especially dancing. That is unless you just want to chill all night, sitting in the green dress will cause some gaposis issues too, so you're going to have to be careful even doing nothing!! If you try them on again, wiggle, jump around like you dance in the fitting room like a crazy woman in front of the mirror, you'll be glad you did, you'll soon understand no bra deep vee and hello girls, how'd you get out here!! = }