Who is stronger?

Darth Bane. And I'll tell you why.

Sidious was the most powerful Dark Lord of the Sith of all time. He could do things that Bane could never dream of doing. Opening Force Maelstroms like it was nothing. He could bend time and space with his lightning. And on a level of deception that was nobody else who could rival him. He even surpassed his master- Plagueis- when it came to manipulating events, and this was the same Muun that started the Grand Plan in the first place. Without a doubt Sidious was the better Sith Lord.

But Bane was the better warrior. This was a man that was honed by war from his early 20s and then took that into his Sith training. He was so masterful with a Lightsaber that he could literally intercept rain during a training session once. He epitomized what it meant for a Sith Lord to be able to fight multiple Jedi by himself. I know that Sidious was supposed to be a Blademaster and had mastered all of the known techniques, but Bane was the better duelist and he was as cunning as Sidious was too. I have no doubt that Bane was on leveled setting with the man that would eventually be the last of his lineage in terms of cunning, in fact. With that in mind he would never let Sidious get to the stage where he could open up the vast almost infinite knowledge he had on Sith magic.

To put it into perspective Bane almost survived a Transfer Essence ritual too. Zannah nearly lost that battle.

Bane > Sidious any day.