I researched and worried when I built my bike last fall. You can find arguments for and against the two wheel sizes. I opted to go with the 700's based on the recommendation of lbs owner. It's hard to know why the effort is so much less on my new bike. I can pedal at a much higher cadence and work much more efficiently. However, I made so many changes and this is a custom built frame that it is hard to know how much of that if any is due to wheel size. For me, I think it may have more to do with substantially upgraded and lighter wheels rather than their size. The difference is incredible.