I definitely say real thing.
The knockoff will probably be made of inferior quality materials, and probably not cut the same - may not fit the same. Not that I think you should pay full price, though. Check out pearlsplace.com, the knot's dress board, ebay, and preownedweddingdresses.com for a used gown. I paid about 5% of retail on my Monique Lhuiller gown by buying it used on ebay.
It would suck to get the dress and have it *not* be what you wanted, or expected to get, ya know?