Outfits aren't sexy--women's bodies are. So it really depends on how your body looks. You want to compliment whatever your best feature is. Sounds like for you it's your hourglass figure. Would you say your boobs or your butt is your better feature of the two? Wear something that emphasizes that.
The black dress
I think they both look great, it just depends on what you're feeling
Especially since you are curvy, go for the red dress, it would look very good hugging you curves, it would look better than on the model. In the first outfit to be honest it's just trashy. The dress is sexy cause it would show your figure and be provocative and sexy but it still keeps you classy because you aren't really showing skin but still outlines your figure and the dress in general is sexy with the slit and plunge neck etc. I would just suggest different shoes than the ones the model has they don't really go with the dress at all lol