Upscale Italian: Zeffirino or Sinatra?
Both have the kind of atmosphere and service that you are looking for, but we have found the food quality at Sinatra to be better after multiple visits to both. The Zeffirino menu is a bit more extensive, but perhaps at the cost of a lower level of execution - we have enjoyed our meals there, but the dishes have been good, not memorable. If you are focusing on upscale Italian for this particular occasion, it might be worth also taking a look at Nove (the Palms) and Circo (Bellaggio) as well - both bring the right combination of ambience, service and food quality.
I agree with QAW here. Sinatra is a step above Zefferino in service, food, atmosphere and consequently, price. I'd choose it any day over Zerrerino. I look at Zefferino as more of a lunch spot. Their Sunday brunch is one of the best in town.
You may also consider Carnevino. Despite its steak focus, it is a great showcase for some of Mario Batali's works. They don't seem to have the inconsistency, service and execution issues which seem to plague Batali's other venue, B&B. The homemade pastas are fantastic as is the antipasto and gelato.
Bartolotta is also amongst the best in town and has more of a seafood focus. Special occasions would fit in quite well here. The fresh "old world" fish which is flown in daily is not to be and the Family Style Tasting menu is one of the more memorable culinary experiences in Vegas.