^Look, more power is always more useful, but Naruto does have the upper hand at the moment for several reasons.
In terms of combat, Naruto and Sasuke are comparable. Naruto's sage mode
But let's look at personal factors. Sasuke's destroying his body by spamming MS and is losing self-control and even good battle judgment in the process. On the other hand, Naruto has even better control of the kyuubi since the Yondy seal-almost-broken incident.
Also, here's the biggest scale-tipper towards Naruto: Itachi gave him some secret power expressly to fight against Sasuke should the need arise.
But I'm not convinced Sasuke is Madara's successor. We still don't know too much of Madara, and all the testimonies he's given us still don't add up. We have the clan wars discussion he had with Sasuke, then there's the claimed siblinghood to Hashirama, then the revelation of the Moon-eye plan. The most sensible part of all this is his association with the Water country, but even that isn't explained well. And at the moment all Sasuke's been used for is to create disruption while Tobi executes his plans.
Also, a little over a year ago I suspected Yondy sealed the Kyuubi in Naruto with the dual purpose of protecting Konoha and also to protect the Kyuubi from being controlled by Tobi again. The kyuubi may yet be an important informational key, but who knows.