Hmm. . . hmm . . . let use resume some facts.
During their life time, as far as Kishimoto has shown us, Madara never even once beat Hashirama in any battle. Madara in some of their battles had used the power of a Tailed Beast against Hashirama, but still he lost. Even from the early beginnings Hashirama was stronger than Madara, which Madara himself admitted.
We know that they are stronger when they are alive than reincarnated. Which means their power was decreased somewhat about the same level when they were reincarnated, except the fact that Madara had Wood Release. . . but even with it he was still loosign to Hashirama (if Black Zetsu didn't force Obito to revive him, Madara would have had been done for).
Alive Madara is obviously stronger than reincarnated Hashirama. Madara also stole Hashirama's Sage Mode, which makes him even more stronger.
Now, think about it: "But what if Hashirama was also alive. . . which means more power?"
PS: Also, one thing - Madara almost always stole power to become stronger (Tailed Beast, Wood Release, Sage Mode), while Hashirama never had that problem, he was becoming stronger on his own. :yeah: