:: Alito is a dorky-looking white guy. Roberts is a dorky-looking white guy, but handsomer.
:: Alito has a wife, a son, and a daughter (and by the way, since we know no White House picture is taken by accident, are they trying to convey Bill Clinton's ostensible blessing on the nomination?). I can't find pix of Roberts's family, but do you even need the visual reminder? He also has a wife, a son, and a daughter, and they're blond, gorgeous, and charming. As opposed to Alito's family, who might be uncharitably described as blond, cranky, and rigid.
:: Alito is renowned for his modest brilliance. Roberts is renowned for his modest brilliance.
:: Alito is a federal judge. Roberts was a federal judge.
:: Alito is so very conservative on issues of abortion, the death penalty and the separation of church and state. Roberts is so very conservative on issues of abortion, the death penalty and the separation of church and state, but only probably.
I could go on, but I think you get the picture: the Bush team pulled out what's appearing more and more to be their dominant philosophy: stay the course. If you liked Roberts, you'll... well, you won't like Alito quite as much, but he'll do the trick in a pinch.