Brian founded the band--put an ad in the jazz paper. And stu answered the ad. Brian wanted Keith in the band, but Keith wouldn't come without Mick. Brian thought Mick's voice was a bit weak, and might not hold up during long sessions. Brian named the band, promoted the band, kept the books, and charted their early musical direction.
Obviously, he was very musically inclined and could get something "nice" out of any instrument he picked up. His colorful flavor added immensely to the Stones' songs during the 8 years he was in the band. When Andrew Loog Oldham started managing the band, he decided that Mick & Keith should start writing the songs. Brian was gradually edged out of the area of musical direction and, due to a number of reasons, delved more and more into drugs and alcohol. I still think he was a very viable member of the band until late in '68. It is said that he wanted to leave the band, and get back to his bluesy roots, as early as 1965, but, Andrew and Mick feared that, as popular as Brian was, the band might fall apart without him. So, he stayed...until June of '69.
Stu helped keep things together in the band. He was vastly important in many areas concerning the Stones. But Brian founded The Rollin'/Rolling Stones. It was Brian's "baby".