Tire design differences too. I like Continental Ultra Sport II -- good and cheap -- and Conti Grand Prix Classics, great affordable skinwalls. They ride very similarly, but are very different in fit.

The Ultra Sport II are notoriously tight fitting. I needed a Kool Stop bead jack to seat them. Whenever I ride with those tires I stuff the bead jack into a jersey pocket in case I get a flat.

On the plus side, Ultra Sport II in 700x25 will clear my Ironman's GPX rear brake bridge, no problem.

The GP Classics (700x25 only) have a slightly better ride (much thinner sidewalls, translucent through the reddish brown skinwalls), but the raised center tread rib would just barely clear and any slight road debris would scrape. After a few rides, including some sessions on the indoor trainer, it scraped less after riding through a wet spot. Now it clears, no problem, and there's plenty of wear remaining.

And no need for a bead jack with the GP Classics. I can mount them with my hands, despite aging joints and arthritis. Very helpful after switching to latex tubes -- less risk of pinching the tube and causing a cut that would be difficult to patch on latex.