Assuming the tires were at identical pressures, your positions were identical, and the geometries were relatively close, I would assume the higher cost of the Giant is due to a higher quality frame.

One of the ways this may be evident is thru increased power transfer. This is achieved by having a stiffer rear triangle. The fat downtube keeps the bike tracking straight and the thin top tube takes the sting off the bumps. The Giant has more manipulated tubes that allow more tuning. A characteristic of thick tubes is a dead sensation, while a thinwall frame would generally be "livelier." This could be what you are feeling. Did you confirm any speed gains or are you just going by gut feeling?

Another thing could be the wheels. Lighter wheels accelerate much faster. But they both have fairly generic wheels