I don´t do lists but I want to say that Duran proved himself in terms of p4p while Ali did not. So, we know that Duran was a great p4p fighter with Ali we can just assume that he was. If I had to chose, i´d pick Duran.
I've thought long and hard about it, and I'm chosing Duran. Ali's lagacy depends on a few fights whilst Durans whole career is brilliant.