They are more modern and the buildings are a lot taller.
Europe has a lot more older buildings. While North America has some pretty modern styled buildings in big cities. Like the Walt Disney Concert Hall... As infamous as it is.
U.S. has modern buildings but in terms of road infrastructure, Europe is better.
Europe has old buildings that are more stable and have lasted. Modern America urban buildings are all made out of wood and could be easily be be destroyed be earthquakes, tornadoes, etc.
It comes down to personal taste, but I like older architecture.
The title says 'in general' meaning most if not all buildings in both regions. So I have to go with Europe, reason being, they don't have a bunch of ugly box buildings everywhere. What I mean are those crap Wal-Mart looking box buildings or the McDonald you see on every street. While Europe on the other hand maintains some architectural dignity. Although old/modern american skyscrapers tend to impress, at least for me.