With the Latest manga (624) we can clearly see that Senju clan is Superior to Uchiha clan. I mean even Uchiha defectors were giving themselves in to the Senju. Even with MS, Madara couldn't beat Hashirama, Just how Powerful are the senju? More importantly Hashirama?
We know that they've got huge chakra reserves. Hashirama's power is certainly the largest but maybe not by a huge amount. We may never know.
What's interesting is to go back and compare to some of the earlier chapters in the chuunin exams, when people talk about Sasuke and the Uchiha as if they are the most powerful clan in the village. Clearly the Senju were more powerful at one point.
It might also have something to do with relative numbers.
Notice that most Uchiha don't awaken their MS, and at least a handful of Uchiha might have sacrificed their own sharingan for performing Izanagi and Izanami. We also know that Uchiha don't awaken their sharingan eyes until they've been in a large amount of battle stress, which means that younger Uchiha are going to be weaker on average than senju children (Senju children have their power automatically and don't need to unlock anything to reach "full power").
Tobirama also talks about how the desire for vengence drives some Uchiha insane.
Add these things together, and it's not hard to imagine that the Uchiha clan reproduced less, had more children die young, and some of their best warriors were driven insane to the point that they couldn't marry and have or raise children.
It might have just been a numbers game. There were more Senju, the Uchiha were outnumbered, and even if they were more powerful individually they would have lost in the end.