User-customizable settings for appearance of blog?

I have (added) code in the theme that I’m using (Atahualpa) that allows me as admin to flick a switch that determines whether comments appear on the main page following each post, or whether only posts appear (without comments displayed). So far, so good.

But I’d like to provide, say, a widget that would allow my users to determine for themselves which style main page they’d like to view.

If I were to create a widget that had a way to select, say, yes or no, I presume that would have to involve setting a variable somewhere. (Alternative approaches cheerfully accepted.)

My question is this: If a widget defines a variable to have one value when set by one user, does that variable then determine the setting for all concurrent users? Or does each user have his/her own thread in which the variables are scoped to the specific user/instance?