This thread reminded me of an article I read not too long ago about how to dress sexy in midlife. It said to do one thing at a time - sexy skirt or pants with a more conservative top, or sexy top with more conservative skirt or pants. That's exactly what I do on those occasions - I never do both at the same time.

On the two dresses - I'd vote for the li'l black dress. It may look a little bit like a slip in the picture but with a few accessories, some pretty shoes - you'd be a knockout.