Philip Wakem is a lawyer's son. Deformed, with a hunchback, his chief persuasive appeal to Maggie is his sensitive, caring, and supportive nature. Philip is intelligent, perceptive, and artistic, as well as vulnerable, all characteristics that draw Maggie to him. He is also in love with her, and reconciled to being in love with her even if she won't marry him. He writes to her that:
I never expected happiness: and in knowing you, in loving you, I have had, and still have, what reconciles me to life
While Maggie loves Philip dearly as a friend, she cannot find it in herself to fall in love with him in a romantic way.
Stephen Guest is in many ways Philip's opposite. He is a wealthy, confident, and, more to the point, a very handsome young man. His persuasive appeal to Maggie is primarily physical. She is sexually attracted to him, lighting up when he is around, just as he is sexually attracted to her. Maggie never knows Stephen on a deep soul level, and it may be that, unlike Philip, there is no deep soul level in this somewhat immature, seductive, and shallow character. Maggie certainly doesn't want him for his mind. Beyond his body, he appeals to her romantic fantasies of what a lover should be.
Neither Philip nor Stephen satisfy Maggie completely: one appeals to her mind and soul, the other to her body. Perhaps what she most longs for is a combination of the best in both men.