Take the fast lens. I've done sports and events, I might use a 50mm for a smaller event or a longer lens if it's a large ballroom. I try to notice where it looks like the light is hitting and get photos in those parts of the room. Of course, that's for an event where I don't want pictures of everybody (forget the people in the dark corners!). I might want room shots or the podium and displays etc.
I just took pictures at a banquet (for my personal use not shooting the event) and used the existing light; had to use a fairly high ISO. I think a restaurant might be more tricky depending on the lighting (might not be overhead lights and the whole room could be fairly dark), depends on if flash is an option in the restaurant (and being respectful of other diners, unless you're in a separate room).
Maybe do some group shots with the flash, some without; try some candids with existing light (so you're not aggravating everybody firing off your flash all night long! lol). Go early and do some test shots of the table etc. with and without the flash, and see what you can get before everybody in your group gets there.
I know of a couple of photographers who just stick a white business card in front of their on-camera flash to diffuse the light (in a pinch) - there are pop on diffusers, the Stofen I think are supposed to be pretty good, but I haven't tried either of these ideas.