Should I use an S chart or an R chart?

Both S charts and R charts measure subgroup variability. The S chart uses the standard deviation to represent the spread in the data and the R chart uses the range.
When to use the S chart
Use the S chart when your subgroup sizes are 9 or greater. S charts use all the data to calculate the subgroup process standard deviations. You should consider using S charts for processes with a high rate of production or when data collection is quick and inexpensive.
When to use the R chart
Use the R chart when your subgroup sizes are 8 or less.

The maximum subgroup size in Minitab's R chart is 100.