Freestyle 2K. Certainly I am faster in this pool, maybe because its being 25 meters means twice as many pushoffs but I am sure that not having to share a lane helps a great deal. Being able to keep straight over the line instead of having to circle means I don't steer into the rope nearly as often, especially with my strongly dominant right side.
But after 2K, my belly is sore. Perhaps I will eventually have abdominal muscles.