Both are judged by "the eye of the beholder", beautiful is almost static and more closely related to looks and sexy is fluid/ motion. A beautiful lady might get a second glance but sexy will get the hormones pumping.
I feel there are many people that might think they have above average looks but have no intuition on how to be sexy. I'll personally rather be with someone that can flaunt it and be sexy that someone that believes their looks make up for everything .