I can't speak about which dealership has the best service department, but I can tell you that Findlay VW in Henderson, NV will definitely be the easiest one to get to. Coming from Phoenix's direction this dealership will not require you to fight much Las Vegas traffic. It is maybe a mile from the highway you will enter the LV valley. AutoNation VW has pissed me off in the past, but I know nothing about their service department. Findlay VW North is just on the far opposite side of Las Vegas from Phoenix and again, I know nothing about their service department.
Personally I try to avoid dealership service departments (except for warranty work) whenever possible, but my limited experience with Findlay VW (Henderson) service department has been OK except for being over-priced (like all dealerships).
Have Fun!
Don