Agreed, avoid the WMATA and stick with Amtrak. The BWI-->Union Station ride is actually quite painless, and if you look for July 4th tickets on Amtrak now, you can get them as low as $11 for a wide array of times. The BWI shuttle to the train station runs every five minutes or so, and if it meant saving $300 there would be little question of choice for me. However, only you can judge how much your time is worth, and if the extra 1-1.5 hours on your travel in each direction is too much, you should just stick with DCA.