Dulles/Manassas is the same thing ... Manassas is the airport and Dulles Aviation is the name of the FBO (School). I picked Manassas just because I wanted to get off the damn base. Nice change of pace. Its bout a 30 min drive but I didn't mind it at all.
Also, the program there is WAY more structured as opposed to Quantico where they had one student that had never landed b4 his check ride. When he jacked up the landings on his checkride and they gave him crap, he told them that this was his first time landing. They had no idea. He needed like an extra week to complete the program.
At Manassas they have it set up by flights and everyone does the same manueuvers on each flight so it is way more efficient and you actually learn more.
Also, at Manassas you'll do a night crosscountry flight (w/ instructor) and night bounces where as at Quantico you won't.
Peace,
-Spin