First thing's first: the dying wishes of any father is to see his children coexist in harmony. The challenge So6p faced was choosing to give each son something they didn't want in hopes they would choose to acknowledge their cumulative strengths and act a unified leadership after So6p died.
Instead, because the two sons felt as if rivalry were more important, they did not unify and become strong leaders together.
I also don't like questions where someone asks 'who is stronger.' Strength is a misleading word because it implies inherent power and superiority. In the ninja world, the 'stronger' ninja is always one who uses his tools in the most effective/efficient manner.
For instance, a ninja who has mastered the use of a Kunai will be able to beat any ninja who has not. The ninja who has mastered the use of a Kunai will also stand a good chance of being more efficient/effective with his techniques than a ninja who relies on other techniques, but can't utilize kunai very well.
This lends the ninja who mastered Kunai a strength over other ninja who didn't.
A perfect example of this would be Naruto after he had barely passed the genin academy exams vs Kakashi who was to be their instructor. At the time Naruto had the Kyuubi sealed inside him, which was capable of unleashing enough power to destroy all of Konoha.
But because Kakashi had mastered so many techniques, he was able to easily counter anything that Naruto could do (while they were doing the survival training to get the bells).
Because Naruto had no idea how to use the kyuubi's power, he was very weak vs just about anyone.