As long as its someone who you know will do a proper job!
You may save a bit of extra cash rather then get ripped off by the dealer!!
Then again, its down to whether you want a full dealer service history with the car.....
I would compare prices, although I never really trust dealers with any car when servicing. You never know what they do with it!
As I said, compare prices, then decide. Does your local guy know a lot about the GTI and what to expect or do?? You dont want to end up taking it to a dealer to fixing more due to mechanic error!