I ride with both. My Jamis Eclipse and 80s Specialize Sirrus, and Colnago Master XL have level top tubes. My Cannondale and Specialized S-Works Roubaix have sloping top tubes. And my Indy Fab CJ TI has a very very mildly sloping top tube (it's not discernible to most people who see it). Thing is when I'm on them I never even think about it.

I do, however, wonder why we call it a sloping top tube instead of a slanting top tube.