I know there's a lot of threads about Hashirama's Sage Mode and I just want to put out my theory out here. Sorry in advance for my english.
I believe that Hashirama Sage Mode is the "ace card" in the Senju Clan just like the Sharingan and MS is the "ace card" to the Uchiha Clan.
So6p older son had the power in the eye - it later became the Sharingan
So6p younger son had the power in the body - it later became the Senju Sage Mode (and the Sage Mode is the power in the body)
I believe that not all in the Senju Clan could use there Sage Mode (I believe that it is in the same rank as the Uchiha MS).
I believe that Hashirama Sage Mode is the "ace card" in the Senju Clan just like the Sharingan and MS is the "ace card" to the Uchiha Clan.
So6p older son had the power in the eye - it later became the Sharingan
So6p younger son had the power in the body - it later became the Senju Sage Mode (and the Sage Mode is the power in the body)
I believe that not all in the Senju Clan could use there Sage Mode (I believe that it is in the same rank as the Uchiha MS).