Sophie's deformity, and thus her danger, is more immediate. If anyone sees her foot, she is doomed. David also has a problem, because someone might realize he is telepathic. His problem, what makes him different, is easier to hide. Therefore I admire them both, because I admire Sophie for dealing with her toes and David for trying to protect her.
This question may be said to have an element of unfairness to it in that Sophie was discovered and exiled. In exile, her values and strengths were modified and altered resulting from the shock she experienced and difficulty she lived in. In the raw novel, David is more admirable, if character traits and behavior are considered. If, on the other hand, Sophie had survived undetected as long as David had (which is unlikely since her deviation is a visible physical one), then it may be conjectured that Sophie and David might be equally admirable. But that didn't happen. Sophie was exiled. As the novel stands, it may be said that David is more admirable.
This is a great question. I hadn't thought of asking this. To be honest, I am not a big fan of David. Although we can obviously sympathise with his position, and the way that the story is told from his perspective helps us do this, he shows himself to be a very passive character, depending upon others to lead him and tell him what to do. He is not the leader of the telepaths, and he is saved more by Rosalind and her preparations than anything he is able to do. He is always guided by Michael and others, and his few attempts to achieve bravery are not successful, such as when he independently tries to double back and rescue Rosalind and Petra from his uncle.
However, if we consider Sophie, I think the reader feels much more sympathy for her and her position than we do for David. In addition, the way that she has managed to carve out a life for herself is very impressive. It is clear from the way that she calmly insists that she must commit the murder necessary to save Rosalind and Petra that she has grown up and matured a lot as a result of the harshness of the Waknukians. The way that she holds true to her love of the spiderman, even in the face of the disgust of David and Rosalind, likewise shows her ability to look beyond appearances. Although she is not a major character, I certainly believe that Sophie is the more admirable.
I feel that Sophie is more admirable than David. This is because Sophie had to suffer the consequences of banishment to the Fringes. Sophie lost her parents at a young age (Parents whom she loved very dearly. This is contrary to David's relationship with his parents. Obviously, Sophie would have felt more lonely and lost without the support of her parents than David would have) and her living conditions in the Fringes were pitiable. Nevertheless, she did all she could to help David, Rosalind and Petra when they arrived at the Fringes, and she even killed to assist them.
We cannot say that David is not admirable because he had suffered a lot due to his treatment by his father. However, he can relate his troubles to his Uncle Axel and his telepathic friends. Also, he was assisted in many ways, from his escape to the Fringes to his journey to Sealand.
David is not exactly an independent figure we can see, whereas Sophie is. She has to fend for herself out there in the Fringes. On the other hand, David, once again had support from Rosalind (Who had packed food for their journey; Rosalind also kills a man who is suspiscious of them)
Hence, Sophie is a more admirable character than David.