For a DB quest, I have the choice between killing Cicero or letting
him live. How does each one affect the game, and is there a better one?

I believe that if you don't kill Cicero, he will become a better guy, and will kinda help you around the sanctuary

How many time is this exact same thread idea going to be posted?

There's a search bar... it should probably be used every so often.

Yes, but not during the quest, do it at the boethiah shrine... perfect sacrifice.

When it comes to killing a fellow guildmate I'd exercise caution, especially when you weren't there when the conflict that led Astrid to tell you to kill him happened. If the DB gets it's orders from the Night Mother, and she didn't order you to kill Cicero, for the sake of following along, I didn't do so.

Cicero is hilarious. Let him live.

If you summon the Spectral Assassin, he'll tell you Keeper is a highly respected position, and killing him is not Sithis' will. That's good enough for me.

Also he makes a damn good follower later... if you can stand that nails-on-chalkboard voice of his.

I didn't kill him because I felt pity for him, yes even as an assassin.

kill- get awsome bartering clothes
don't- get eccentric jester as a companion and here WTFs from your guildies.

Let him live the guys funny as hell