I didn't want to say it, but i guess it safe to say i told you so about the power of kyuubi. As i say before, "Kyuubi has a head and nine tails (doesn't calculate the fangs and claws because all demon has them, nothing special) that bring total of ten. As new level reach, more part of Kyuubi come out (like Garaa) and grants its User more different power." As far as we see, it indicates just that. At the beginning of series, when Naruto activated Kyuubi power, only the head came out and granted Naruto with super speed and strengh. When he fought with Sasuke, level two, a head first, then a fox with with one tail come out. Beside more strengh and speed, charka can move by its will and protect Naruto. The Kyuubi has the most tails; therefore, Kyuubi is the strongest or at least grants the most ability or power to its user. This is also explain why Itachi doesn't want Sasori to steal his Jinchuuriki (Naruto and they Kyuubi).