I actually have this situation. My black Fuji Team Issue is a carbon fiber 17lb bike with full Dura Ace drivetrain. My Dahon XX Anniversary Helios is also right around 17 lbs.

They're both excellent bikes and fun to ride. I have not done "scientific" tests on them for performance difference, but here are my anecdotal and honest thought about their differences:

- Flats - Probably very little difference. Although, I feel that 700C wheels roll more efficiently at higher speeds due to the higher angular momentum. Also, the Dahon's top gearing was around 95 gear inches with its stock setup, so it was a little disadvantaged when the paceline is going faster than 25mph or so. But, that is easily remedied.

- Climbing - As I said earlier, I actually do believe that smaller wheels are better for climbing,due to the fact that it spins up faster. My Rolf wheelset on the Dahon weighs 995 grams (!), the Easton wheelset on the Fuji weighs 1550g. Factor in the angular momentum impact and one can certainly see how it is harder to spin up the big wheels.

- Descending - I definitely feel more stable on the 700C wheels than the 406's. Maybe it's just my poor riding skills, though ..

- Sprinting - The Fuji wins again, I'm afraid. The hinges at the handlebar and mid-frame take a significant toll on rigidity when stomping on the pedals and pulling on the 'bars. There's no way around it. On a one-piece full size frame, it's a very confident feeling when I stand and pull on the bars. On the Dahon, particularly the Helios design, I just cannot pull as hard.

I have been on a two year search for a "portable roadie" - a folder than can keep me in the hunt on my weekend rides with the group. However, I have abandoned that idea because I simply have not been able to find one. That's why I put the Dahon up for sale and it is on its way to a new home. Maybe the Moulton TSR will come close, but it will weigh 24lbs.

Don't mean to start any wars here, just noting my honest observations about the bikes.