The key thing is whoever does it, make sure they do it to the proper service schedule – ie, the spark plugs, fuel filter and cam belt actually get changed every 60k or so rather than just doing “oil and filter” at every service.
When I’m buying a car, specialist history looks better than a random garage, and will probably help sell the car given it’s previously had full Audi history. If the book is full of random stamps, it’s not going to make much difference who does it.
As for whether specialists are worth it or not, depends on the car and the specialist. The bloke who fixes my wife’s Alfa isn’t cheap, but whenever it comes back from him it runs spot on.