This is a complicated issue, If you look at the trend, the O's attendance has fallen every single year of this decade, which obviously has nothing to do with the Nationals, until 2005, when perhaps, it fell a bit faster.
On the other hand, it is indisputable that people situated like me, in southern Frederick County Maryland, are equidistant from both franchises. In the past, I only had the option of attending O's games, now I have a choice. Obviously, that will cut down on some attendance.
Nevertheless, it is a virtual impossibility that the Nats' new stadium will be as nice as Camden Yards and I believe that it will still be easier (not to mention cheaper to park) to continue driving to Baltimore, than trying to schlep to SE Washington for people living in Montgomery, Frederick, and Washington counties Maryland.
Ultimately, its the product on the field that will make the difference, however, since people rarely root for two teams in a serious manner. Since arriving in 2005, the Nats have been scrappy overachievers who are fun to watch. The O's are the exact opposite. Unless Baltimore can reverse this trend and put winning baseball on television in order to draw fans, their wonderful ballpark will not be enough.