To kill Cicero or not to kill Cicero

He has 500 gold on him as well as his gear. Since you can pickpocket him and receive higher grade gear at the Dawnstar Sanctuary, it is really not lucrative to kill him. If you have completed the quest line for the Dark Brotherhood, and Cicerio is still alive at that point, he will indeed become a Follower.

Leaving him alive thus results in more benefits for you...

I just kept him alive because if I really wanted to I could just kill him later anyway. However, I don't know if it changes the storyline at all. When I met Cicero, I chose to help him, but I never got him arrested, so if you didn't help him and he ended up showing at the sanctuary then I don't think it really matters what you do since you still get the same results

You don't get another chance to kill him. After the quest if he's still alive Cicero becomes essential again.