Actually I use 650c tires. I ride a 47 cm Trek. I think Trek's road bikes sized below 50 cm only come with 650c tires. Do a search for 650c in the forum and you'll find a quite a few discussions on the topic. I only learned the difference a few months ago in a search for backup tires and tubes.

For more info, also check out. http://www.sheldonbrown.com/harris/tires/index.html

From my perspective, I can't compare 650c and 700 tires. I haven't had the chance to ride on anything other than 650c. I can say the tires and tubes are a bit harder to find. There's plenty online though.

-Shimpie