I used 23mm gatorskins for a long time. My new bike came with some other kind of tires that were 26mm. Maybe I'm inattentive but I don't really notice a huge difference (or maybe it's because I spent so much time figuring out the brake and seat post problems.

However, I will mention that it's been much easier to use the folding tires than the the non-folding tires. I once spent 30 minutes at home trying to get new tires on and finaly took the wheels to the shop. My hands just weren't strong enough to get them over that set of rims. Or if you're switching out the tires anyway. They cost a little more but they are so much easier to handle.