Select both as we want to get details on the long as well as the short paths. Slack reports indicate time constraints by providing the the difference between the required time and the signal's arrival time at the path end point. Slack time can be more critical than a long path delay that still meets the required time. Slack time can be negative. Sorting by slack time is the default.
Arrival sorts according to arrival times at end points.
Feel free to experiment by generating reports based on the slack time and the arrival times. The end meaning remains the same. Hit apply button. Ignore the trade-off curves tab. This will make Synergy to synthesize the design as per the constraints provided.