oh, not on the same scale but there is an LV wallet I see on ebay at the moment for less than it would be in the boutique and it's with a seller I've purchased from in the past and been happy with.
The smart thing would be to get it preloved because it looks like it's in fantastic condition and normally I have no problem going that route, but the other side of me really wants the boutique experience and walking out with a new purchase on my history because it's been 2 years and I've been very good about purse purchases... (jewelry, not so much).
But from an AD, your watch would still be new? You're not going the estate route? If new in both places, I'd say money saved is money earned... but ultimately, go where you heart is.
edited to add: Our local Costco has a Cartier Ballon bleu and a tank, too. Less expensive, very authentic, but zero on the "Cartier experience" side....