Exactly. The two clans have been rivals for centuries and have always been equally strong. Senju have strong chakra and the Uchiha have the Sharingan. Only Madara and Hashirama were special. Madara was the first to awaken the Mangekyo, Hashirama possessed Wood Release. Saying "Hashirama defeated Madara so Senju are stronger" is pointless, is doesnt proof anything! We are talking about the clans, not two induviduals. It doesnt matter that these men were the most powerful out of their respective clan because just from that you cant judge which one ist more powerful.
The Uchiha were definately not stronger, we know that from what Tobi told Sasuke.
And if the Senju were stronger, they wouldnt have made that peace contract, why ally yourself with an enemy that you could defeat? And every Uchiha except Madara wanted to accept this contract, so they were not able to defeat the Senju neither.