I got engaged when I was 26 and I tried on both and went with the classic Tiffany setting. The Lucida is stunning as well but it's a bit harder to pull off. I also found you needed bigger stone to make the Lucida look good (IMHO) and I think you would have too much metal, especially if you're looking at a 0.5 carat.
I would go with the classic if you're getting a stone that size - it'll be very pretty and feminine.
FWIW, I'm now in my early 30s and still love the style as much as I did when I first got engaged!