The guitar would be slightly harder to start off with, but the bass is actually a little bit more difficult when you are a bit more experienced than a newbie. Overall with the real hardcore stuff the guitar is definitely harder. Theres a lot of modes, scales and techniques to learn, and they appear a lot more often than bass solo as such. If you wanna learn the guitar, start off by learning all the easy chords: E, Em, A, Am, D, Dm, G, C, B7, G7. Then you would wanna learn Power chords and Bar chords Then learn pop rock songs and learn to count. Then you might wanna try some easy solos.