Sidious, in my opinion.
He can consume the essence of a whole planet, manipulate anyone and wrap the existence.
But it's like comparing apples to oranges. If we compare the limits of their power, it'll be probably the one can affect a larger scale.
In the other hand, a force user doesn't need a tool to use the force. A lightsaber is not used to use the force, it's a thing that can be useful against a force user and it can be put to a good use by a force user with their connection to the being.
We all know that there are many restrictions apply to everyone who are dealing with the usage of a spell. A Sith can break the chains that are holding them in both philosophy and in their own line of reality.
A Sith is both deceptive and fierce, they doesn't stop or doubt at any point. Vader, he is not a real Sith. He is a slave to the Sith.
But it's not really a good thing opposing two different lines of realities.