Since the question is which mode, the Sage or Rikudo, is more powerful, I would have to say the Rikudo mode is. Sage mode has many advantages. For instance, Naruto utilizes Senjutsu chakra (natural energy) which increases his speed, strength, and endurance. This allows him to not only perfect perfect and reuse techniques like the rasenshuriken but also to have a cloak of chakra that can cause major damage to enemies, seen by the attack on the Preta path of Pain.
On the other hand, there is the Rikudo, or Nine-Tails Chakra, mode. This mode also increases the speed, strength, and endurance of Naruto. Since he is using the Nine-Tailed Foxes energy, Naruto is able to lift a number of boulders with his chakra alone. His speed is seen to greatly increase when he is seen crashing into a wall. His speed is compared to Minato's which is incredible. Though his endurance may be limited to how much is consumed by the Fox in the Rikudo state, when it comes to power, this mode is impressive. Rikudo mode even has a cloak made from the Nine-Tail's chakra which seems more powerful than Sage mode. In the end, it is unsure of the full extent of the Rikudo mode but features as stated above along with the simple fact that it was introduced after Sage mode makes it more powerful.