After a really keen observation as to how Susano'o forms for the individual uchiha, I came to know about 2 things:
1) The color of the susano'o varies on the amount of hate/love of the user. Like Itachi, his susano'o is red-orange symbolizing happiness(or something, lol) whilst Sasuke's is dark purple symbolizing hate.
2)The weapons initially formed by susano'o are the weapons the user of susano'o first came in contact with in his entire life. Like Madara, the first probable weapon he came in contact with was a katana or sword, Itachi being with Kunai(the magatama rings) and Sasuke the bow and arrow.
Different conclusions are reached when one fact is viewed from two separate points of view. When that happens, there is no immediate way to judge which point of view is the correct one. There is no way to conclude one’s own conclusion is the correct one.
About the Magatama, that's not true at all. If my memories serves me well, Magatama is a kind of amulet/ornament (related to Shinto) that wields spiritual power. It doesn't have anything to do with a kunai or shuriken (Although Itachi uses this technique as a throwable jutsu, then it's only a projection of his spiritual powers).