I was never a fan of Sydney, I don't know why. To me, he seems to be self-absorbed and cynical. I'd like to read his bio, though.
I had always thought that Syd was the more easy-going, level-headed brother, but all that changed after reading his bio (which I highly recommend). He and Charlie were actually cut from very similar cloth. Like Charlie, he was prone to depression. Both were paranoid and distrustful of people. Syd also had an interest in young girls (one unfortunate event actually ended his career). However, I don't believe there would be a Charlie Chaplin as we know him today without Sydney. It was Syd who recommended Charlie to Fred Karno, which eventually led to his job at Keystone. He was also instrumental in negotiating Charlie's first million-dollar contract with First National. If you can, you should read Lisa Stein's bio, it's well-written and carefully researched. Several of Syd's letters to his friend R.J. Minney are included in the book. They provide great insight into his personality, as well as his thoughts and opinions about things--including his interest in nudism (!)