Depends on image (which we would be happy to see) it may be angle of peek, average height of peeks on eg. half of full range of the image height, etc. It is really difficult to talk about picture without seeing them :)

For example, in DOI:10.1111/jmi.12613 I took into consideration the fill of images. Take look - maybe this will give you some ideas how to solve your problem?

Here an example of the three behaviour. I am new to AFM and I don't know a lot of things about its data elaboration. I'm using Gwyddion. Trying various elaborations in "statistics" I've found that "slope distribution" could be interesting but I don't know if it is giustificable.

Keep in mind that it may not be necessary to quantify how continuous the surface is, AFM in the end is a photo.

That said I do not know of a way to quantify the difference you see but approaching the problem as finding a way to quantify how 'continuous' the surface is in terms of mean area between discontinuities may work. This can be done by a way of particle counting and reporting mean size of a 'particle'.

Good luck!

Looking at picture I got some ideas:

1) try to count variance of all pictures (same scale X, Y, Z), eg. with imageJ

2) maybe histogram will show the way?

3) as I remember, this may help: https://www.researchgate.net/post/Fourier_transform_of_AFM_images-what_does_FTT_show

4) roughness parameters?

5) particle size and distribution?

Hope it helps