Perhaps people haven't realized it due to the difference of their personality but their paths are scary similar.
Madara wanted to be hokage instead of Hashirama
Orochimaru wanted to be hokage instead of Minato
Madara left konoha because he wasn't hokage
Orochimaru left konoha because he wasn't hokage
Madara fought the hokage
Orochimaru fought the hokage
Madara is immortal because of his EMS
Orochimaru is immortal because of his white snake
Madara want's sasuke's eyes
Orochimaru wanted sasuke's eyes
Madara took over mizukages place
Orochimaru took over Kazakages place
Madara was a mizukage for his plan
Orochimaru was a kazekage fos his plan
Madara wanted to protected his uchiha family long time ago
Orochimaru wanted to save his family long time ago
Orochimaru hates uchiha and just want their power
Madara hates uchiha and just want their power
Orochimaru wants revenge against konoha
Madara wants revenge against konoha
Orochimaru puts up with Sasukes rude behavior
Madara puts up with Konans rude behavior
They both seek greater power and their masterminds..
Who is a greater villain?...
Isn't that obvious? Orochimaru is the kind of villain you only see once in a couple of decades. Of course this is only my opinion.. I think Madara lacks: Tenacity, desire, complex personality, dement mentality and the scent of being a total alien when not really.. Madara is just enjoyable to watch him do his actions and manipulate but hes just not Final villain quality in my book.
So, with all the similarities in their paths threw the manga, it seems like there isn't enough room in the manga for both of them.