What is the difference between the two?

The only difference is in the number of Active listings on each report.

The Market Conditions (Cumulative) Report displays the number of active listings for that entire period. If a listing was active for even one day during any part of that time frame, it will count on this report.

The Market Conditions (Snapshot) Report displays the number of active listings on the last day of the period. This corresponds to another report that appraisers use, so this is the report that works best for them.