Ben, the mechanics you are talking to are full of shit. A turbo running at the same amount of boost will always make more power than supercharger on the same engine.
Lag is really only a problem on smaller engines, and dont let people fool you, supercharged engines also suffer lag also.
Maintenance, if the Turbo setup is decent, it wouldnt cost any more to run than a Supercharged car.
But the drawbacks are cost, a decent turbo setup can cost 30-100% more than what a supercharger setup would. Plus like I mentioned before, heat is problem with turbos also.