I'll point out that there is no 'Artistic' Skill. There is, however, an Artistic trait that helps with anything art-related (writing, musical skills, painting, and the like). Unlike in TS2, there is no Creativity skill. Painting with an easel raises the Painting Skill, playing guitar raises the guitar skill, writing novels raises the Writing skill, and so on and so forth.
As everyone else said, in the Symphonic branch of the music career, you will need to have both the Logic skill (from levels 8 and up) and the guitar skill.