I would agree that the Empress has more sex appeal--though, of course, it really depends on what you mean by sex appeal. Different people are attracted to different things.
The HPS often takes on the role of Maiden or Crone. So she can certainly be the sexual young woman, mysterious and very powerful as well as fresh and intense. HOWEVER she can also be regarded as a chaste Maiden, a nun. Or a knowing old crone. These make her sexual appeal ambiguous.
If we accept her as being represented by the Moon, then we have the dark moon as well as other phases. This makes her moody, unpredictable, her sexuality as likely to drive someone to lunacy as poetry. Thus, she may have very powerful sexuality, but her lover might well find this attraction a very dangerous one. It's all too easy to lose ones way there in the dark.
The Empress, on the other hand, is all about creativity and abundance. There is a great deal of generosity in her. We can see her as one who gives of herself, who is creative and selfless in her sexuality, and who has enough to keep on giving. She will, likely, be constant and supportive.
If we accept her as being represented by Venus, then her sexuality is pretty well set as she is a represenative of love. She might be a bit too motherly at times, perhaps even possessive of her lover, but she will keep him/her satisfied.
She is more likely to be the woman with the Rubanesque figure, curvy, lush, all woman and willing to show it. This, in contrast with the HPS, who, depending on the phase of the moon, might hide her femininity behind a veil--which, to be fair, could be more alluring to some, but there you go.
And Lyric is absolutely right that there is that contrast between the earthy woman (Empress) and the distant Moon Maiden. You don't have to woo the Empress down from the sky, and she certainly knows what she's doing and how to make it fun.
Does that answer the question?