Should I write: "areas becoming denser" or "more dense"?

I am trying to describe how cities have been affected by the growing population in terms of the density of bodies.

This is how I have it at the moment but I am unsure whether it should be "more dense", or "denser"

With the global human population increasing and areas becoming more dense

or

With the global human population increasing and areas becoming denser

Which would be correct?

Both of your constructions have problems. Strictly construed, you are writing about the density of "areas," while you are clearly thinking about population density, not area density. Moreover, it would be impossible for population density not to increase in the face of increasing global population. Perhaps this will work better for you: Along with the increase in global population, certain areas are showing a disproportionate increase in population density.

We usually talk about a population's density ¹, not a city's density.

To use the comparative form, I'd say that A has a higher (population) density than B.

Japan, with the highest population density, is worst afflicted

In areas with high population densities (c. 100 people km-2) intensive dryland agriculture is becoming increasingly important

The opposite of high (population) is "low"

from the 1981 census data, and it shows the low population density and the extent of areas that are effectively uninhabited...

You can talk about a city having a dense population, in which case the comparative form is denser, and its superlative form, densest.

Whereas the comparative form, more dense,² is used when comparing the density of two different substances, materials or liquids.

Water that is salty is more dense than water that is fresh.

Note: All the quoted texts come from the website J.T.W (Just The Word) which makes use of the British National Corpus.