"There is no excellent beauty that hath not some strangeness in the proportion."
I think beauty is a lot more divisive than pretty. For example, a girl who's a 8 or a 9 - 95% of people would say she was pretty. But for a girl who is strikingly beautiful, some would rate her as a 10 but others at as a 3 or 4. I think there has to be something unusual in the face to take someone from "very pretty" to "beautiful"
For example this girl, who supposedly has a perfect face. I think everyone would agree that she's extremely pretty. But personally I wouldn't describe her as beautiful or striking.
Women like Eva Green, Angelina Jolie, Keira Knightley, Berenice Marlohe I consider beautiful - as do most people but some consider ugly. Kristen Stewart is another example, she really seems to divide opinions.
And you can be sexy, without being all that attractive. Daniel Craig - face like a mug. Sexy? Hell yes, because of the attitude and the figure.