I reckon it would make sense for Obama to appoint a woman to replace Justice Souter. For nigh on thirty years or more, more than half of law students and new lawyers have been women. Women should be represented on the court, and it is doubtless an artifact of sexism that at least three or four justices aren't female.
Women live longer than men, so a female appointee could be expected to last longer than a male.
I would advise Obama to look beyond the usual pool of federal appellate judges and find talent anywhere he can. Having been a judge before is not really all that important, if the performance of the former appellate judges on the court is any indication. Let's have somebody with some real life experience or a political career for a change, not some crusty ideologue. How about somebody who has actually worked as a lawyer?