A recent LA Times article said that Silverton has started taking Sundays and Mondays off. (In fact I saw her eating at Angelini Osteria a month or so ago on a Sunday night.) I'm sure she'll be there on a Saturday, barring unusual circumstances, but I wouldn't worry about it either way, I'm sure the kitchen has gotten to the point where they are able to maintain full quality even when she's not there.
The menu is the same lunch and dinner; and everytime I've been there, Silverton has been present and hard at work. There are two bars -- it's more fun to sit at the one where Silverton is helping assemble the pizzas, but the other bar is fine, too. (I just don't love sitting in those few seats right next to the door -- it can be tight when a crowd builds.) As for sitting at a bar or a table, it's usually so busy that we tend to take whichever comes up first. Either is perfectly lovely. Have fun!