Well, same difference. Souter -- appointed by President George H.W. Bush -- has been one of the most predictably left-wing (and intellectually undistinguished) votes on the Court (thank you, John Sununu).
Sure, a new member may change the "chemistry" of the Court, but it's not likely to change the outcome of many votes. It doesn't strike me as likely that Chief Justice Roberts, or Justices Thomas, Scalia and Alito are likely to be "seduced" by a charismatic new liberal in the way that Justices Souter and O'Connor were, early in their careers, by Justice Brennan. (Anthony Kennedy, of course, is anyone's guess.)
It's been relatively well-known that Souter was ready to retire for some time. In a final graceless move, he apparently wanted to make sure that the son of the man who appointed him wouldn't have the chance to appoint his successor.