The difference between these two guys is that Rashad is improving. His ground skills are obviously getting better and if he starts to learn some boxing and some submissions he could be a real force. Let's face it, the UFC doesn't really have that many great lightheavyweights. I think that unless Rashad has to fight Chuck, Tito or Vera he'll keep his 0.
As for Diego, I never thought he was that strong of a fighter. He suprised me by beating Parysian, but I think now everyone is taking him a little bit more seriously. I really hope Riggs bulldozes him, not because I hate Diego, but because I think Joe doesn't get the props he deserves. Either way, eventually Diego has to fight Penn, St Pierre or Hughes.
In my opinion, Diego would get destroyed by a lot of the better welterweights in the UFC: guys like Drew Fickett, Tony DeSouza. Even a relative unknown like Anthony Torres has a shot at taking away that 0.
Diego goes first.