Very interesting replies. Thank you. Let me respond to some of your ideas. The difference in the number of gears doesn't seem like a likely answer to me as the range of gear is the same. In both bikes, the highest gear is 48/34. The tires and wheels are really similar so it's hard to imagine that as the difference. While they both have aluminum frames, perhaps the Giant frame is stiffer and perhaps that's the difference. It's the most plausible suggestion. I guess the take-away lesson from this is that biking building is art as well as science - just because the specs are the nearly identical, doesn't mean the bikes are. So studying specs and geometry of a bike can only take you so far.