It seems as though Kishi is categorizing MS/EMS patterns to sword and blade types. Chokuto, Tachi, Katana, just look them up.
The shape of the blade changes the users battle style, so I can only conclude that the shape of the sharigan pattern determines the offensive and defensive techniques that go with it.
Sasuke’s is straight. Among other things, it could very well explain why his Sussano is capable of firing arrows.
The pattern may reflect the user by complimenting his fighting style, it could be hereditary, or perhaps it's random. However since your sword is considered an extension of your soul, I can only conclude that the pattern is more of a reflection of the user.
I could be dead wrong, but an explanation like this seems plausible.