I have the Tiffany classic round brilliant and I love it. Before I got engaged/married, I was deciding between the same two rings, and I am SO glad I decided on the round brilliant. The Lucida is certainly a beautiful ring too, but the diamond is square-shape, so not quite as sparkly as the round brilliant and has more metal, so definitely a different look. When I tried the Classic 6-prong on, I couldn't keep my eyes off the diamond, as it was center-stage, but when I tried the Lucida on, I thought the heavier setting detracted from the diamond a little bit.
You definitely can't go wrong with either ring, but I STRONGLY recommend the round brilliant. Unless your girlfriend specifically mentioned that she wants the Lucida, definitely go with the RB 6-prong. You will not regret it...It's the more popular ring, and is the ring that I think most girls swoon for.
In response to your age inquiry, I was 25 when we got engaged. It's only been 2 years since then, but my preferences haven't changed. I think the RB is fantastic for people of all ages.
Hope this helps.
Good Luck!