[Per Wikipedia](https://en.wikipedia.org/wiki/Spatial_variability) on to quote:

>Spatial variability occurs when a quantity that is measured at different spatial locations exhibits values that differ across the locations. Spatial variability can be assessed using spatial descriptive statistics such as the range.

In accord with the above, a sampling scheme that may assist is as follows: 

1. Draw a horizontal diameter (an x-axis) for each circle. 

2. Fit a simple linear (two-parameter) regression model (Least-Squares of Least-Absolute Deviations) to predict the x-values (dependent variable) versus the # of the point in the sample (independent variable) as they occur and happen to fall on the diameter. 

3. Select one of the usual goodness-of-fit metrics for the regression model.

4. Rank the circles based on the chosen comparative statistic. 

I would argue a valid spatial variability analysis based on the definition provided above, as to quote, "a quantity that is measured at different spatial locations exhibits values that differ across the locations".