Some people find city maps easier to conceptualize because most of us live in urban environments. We know how streets work and have an intuitive understanding of where we'll find businesses and residences, and so forth. Overland maps, on the other hand, require us to design based on principles we probably aren't as directly familiar with: geology, climate, economics. That doesn't necessarily mean that the world maps are harder, because some people have an easier time creating based on things they think about deliberately, and trying to think your way through a city design can lead to a great deal of complexity.

That's all about design, though. There is also the matter of execution. As has been mentioned, you can make either kind of map as complex or as simple as you like. The tendency, though, is to try to make city maps very detailed, with every building shown. That can take ages, but it's not the only way to approach it. A simple district map with only a few points of interest highlighted can be quick and simple to create.

So, as with all things: It depends.