After visiting Christmas Town at BGT, frankly I don't think it's a huge issue at all. The scope and execution just made it feel like a complete let down. Unless SFA can pull off a great event with competing features, I don't see it hurting BGW at all. People go to CT for three major things: The scenery, the shows(and well put together productions like the santa photos), and lastly the considerably above amusement park quality food and beverages. Unless Six Flags really out does themselves here, I don't find it appealing to eat chicken fingers in the cold while watching the local cover bands play cliche Christmas songs, or even worse copyright friendly "originals", with scenery that looks like that one neighbor's house.
This event will be a success for the fledgling park. IT will continue in it's tradition of being the easy, just down the road option for amusement to the millions of people in the DC metro area. Customers for CT that were previously willing to make the trek to Williamsburg, will continue to do so in most cases(or so I foresee).