Dershowitz has been trending towards the right, whereas Finkelstein is a thorough-going leftist. This is not because of his oppostion to more and more holocaust reparations, which is, in one form or another, a common opinion among Jews, but rather because of just about everything else Finkelstein writes.
I haven't read Dershowitz's new book, but if it is true, as Finkilstein says, that it follows Joan Peters' way of thinking, few on FR will have much reason to object.