I made a similar comment in another thread recently, but if I were you, I would pick Enterprise - not for speed, but because Enterprise has you speak with more people in the process of picking up your car... that gives you the best option to ask for what you want.

National and Enterprise share the same fleet of vehicles - with the same vehicle classifications. I've had National rentals where they pulled a car from Enterprise, and Enterprise rentals where they pulled one from National. National cars are typically lower mileage/newer, but for these classes - in my experience there isn't really a difference between the two.