Let's say that I have two places with the same climate and the same geography and everything. The only difference is that one civilization is nomadic and the other has settled cities on the land and practice agriculture.
It is a medieval or renaissance era.
How many people are living in the sedentary territory compared to the nomadic territory?
And are both areas filled to capacity with humans? Do they have draft animals to pull farm equipment and carts of produce? There are many factors to consider.
As a rough estimate, take a roughly self-sufficient island population in modern times but with relatively primitive agriculture. Say Flores, in Indonesia.
Mind that this has pretty much optimal climate for agriculture but is mountainous, so you're limited to what crops you can grow.
The sea also provides a lot of food in the form of fish.
Population, roughly 2 million. Area, roughly 20.000 square kilometers. That leaves you with 100 people per square kilometer.
Flores of course has some industry, but it's by and large an agricultural society outside of the cities.
Compare that with central Kalimantan (Borneo) which has more of a hunter/gatherer society in general (though certainly not exclusively) combined with a similar climate.
It has a surface area of 150.000 square kilometers with a roughly similar population of about 2 million, for 0.75 persons per square kilometer.
I don't know if the land could support more people, but the difference is striking.
Even were Borneo to support 10 times as many people, it's still have a population density 10 times lower than the agricultural/industrial society on Flores.
These are of course rough estimates only. Flores has a net excess of food (iow, it exports food) and its population is mostly restricted by available land. Being an island in a very rich sea, it has a large amount of sea food all around, the area of Borneo described is land locked on 75% of its borders (though it has rivers and lakes).
But it's a starting point. Similar climate, roughly similar geography, 1% the population density for the nomadic/hunter gatherer society as compared to the industrial/agricultural one.
I would say that, on average, a nomadic society would have a lower population density. After all, by definition, they wander about (well, perhaps "wander" isn't the best term). The point, though, is that nomads take advantage of natural resources - open land, for instance. For example, Mongolian nomads used open land to graze their horses. Plains Indians (if you'll pardon the phrase) often followed herds of buffalo. But when the buffalo weren't there, they weren't there. And when the grass in a certain region wasn't good, the Mongolia herders weren't there.
In a "settled-down" society, however, things would be different. People would take all the land they could get, for farming. Also, cities would spring up - greatly increasing the population density. You wouldn't see cities in nomadic societies!
