If Ginobelli's hot then SA beats almost anyone in a playoff series. It's clear, he's the alpha male scorer/defender on a fine team. On the other hand, if he's slightly injured or having a B-game, then it's a toss up whether or not SA advances a round.
I think in comparison, the Celts have no single weakness, besides everyone being injured at one time. Thus, if KG plays his routine D and doesn't score (like his former self) and Shaq/KP does paint patrol and then either Paul, RA, or Rondo does some scoring, then the Cs can advance any round, since all members have an even scoring load for the squad.