Which benchmark math test should I give?
When you give a benchmark test, you are just seeing if your students are functioning at their grade level or not. These tests are screeners and don't tell you what grade your students are functioning at, just if they are functioning on-grade or not (which is at the 50th percentile).
You can decide which math benchmark test to give based on what would work best for you/your students. In either case of test choice, if a student scores below the 50th percentile, you would go to the progress monitoring measures (not give another benchmark test) and start with the proficient math if the student is not to far below the 50th percentile or go to the basic math if they are performing far below grade level expectations.
The Basic Math tests are written to be more easily accessible (fewer cognitive demands for processing what is being asked) and to assess a more foundational understanding of math. Thus, they are most appropriate for use with students who are really struggling and are performing below their grade level peers in the area of math.
The Proficient Math tests are designed to be more challenging for students, assessing a higher level of mathematics knowledge. The Proficient Math tests may actually be a bit too challenging for students who may perform below average. Here again,teachers may find it useful to use the Basic Math progress monitoring measures until students make sufficient progress to perform at or above the 40th percentile before switching to the Proficient Math measures for regular progress monitoring.