Something that complements her best assets and has a color which is becoming of her. It doesn't have to be anything risque or too revealing. In the professional world I like women who wear slim fitted suits as opposed to skirts, blouses, etc.. In a casual environment, slim fit jeans, stretch pants, shorts, or a nice fitting skirt.. In the gym I'm all about yoga pants, spandex, etc.. It just depends on the individual.. Not everyone can get away with super tight or super loose, or any color.. you have to find what works for you.
I think different people like different styles on a girl. For instance my gf has a slack dress and on the beck of the dress it looks like a skeleton spine and ribs for the top 3rd of the dress. I think it's sexy because I'm into metal and stuff like that.
So to answer your question I think it's more of having a unique style to match personality. Something that says something. Not that you should judge a book by its cover.
Both are sexy, but it depends on the kind of attention you want and the type of men that are looking.
I dress according to what I like. I'm curvy but i don't like to show a lot of skin. I tend to keep myself covered. I carry myself like a Queen lol. I feel that is way more attractive than just wearing a skimpy outfit I can't move well in, although on occasion, I dress up more. Your outfit, can tell someone at times how much you respect yourself, but your confidence is what shows it.
Confidence is sexy. Self-worth is sexy. Self-respect is sexy.
Definitely the number one thing is to have confidence. Second thing is if the clothes show your curves and if you have a good body. Third is the outfit its self and how its look on you and with your skin / hair tone