Marion was a more well rounded player but definitely not the better player. Nobody would draft Marion and build a franchise around him like they did Amare. Nobody.
I don't know how I'm supposed to say Stoudemire was the better player with his great deficiencies. All he did very well was score.
But who to build the franchise around... Actually, in my opinion, the franchise wasn't built around
either of these players. It was built around Steve Nash after 2004. (And from the departure of Jason Kidd until 2004, it wasn't particularly built around anyone.) So instead of considering who you would build the franchise around, try this: which forward was more valuable to Steve Nash's teams? Because Nash was the leader and focal point.
I say Marion was, because he did more: rebound very well, defend very well, and score. He scored less than Stoudemire but was no slouch. I see Marion as the defensive anchor of every Suns team he played for. He was indispensable.
Again, I acknowledge these things with discomfort, because I don't particularly like Shawn Marion's personality, for his failure to control his ego. (I can see this as another black mark against Mike D'Antoni: failing to get those two to ever cooperate and develop a good rapport.)