also, barboncino is another nice neapolitan place.
for a really good NY slice, you would do well at Best pizza as well.
Mark, if you have the time, I'd certainly take a shot a Barboncino and Best, but on our last tour, I was underwhelmed by both- and I was not alone in my opinion. Ron (Barboncino owner) is a really nice guy, and it's an authentic Neapolitan pie, but I don't think it's up to par with Keste, Motorino and Forcella.
I don't know if this has always been the case, but insider information this year revealed that Best is a same day dough and suffers a similar flavor deprived fate as Difara (although Difara, with their couple hour dough, is considerably worse). I don't know Williamsburg's dough protocol, but, between Williamsburg and Best, if there's a chance of getting the kind of multi-day flavor that you get at home, the only hope is Williamsburg.
P.S. I just noticed that you're coming into the city on a Sunday. Yeesh. A lot of people leave the city during the summer on weekends, but Summer really isn't in full swing yet, so the city is going to be a mess. I think you'll be lucky if you make it one place.