Pat's is absolutely disgusting. Tony Luke's or John's Roast Pork are the best shops accessible from center city.
Roy's on 15/Sansom has a nice room upstairs and they have a $30 prix fixe menu. They can also be reserved at www.opentable.com (not sure for about 15 people).
Another moderately-priced place that would be open on Sunday is Penne at the Inn at Penn. They have a semi-private room in the back. This is up in University City but a quick bus/taxi ride up from center city. Something for everyone and very moderate prices.