The fundamental difference between Strobe and PurePath is that PurePath is a 24/7 monitor. It captures every single transaction that executes and reports on that transaction as it executes from the browser to the mainframe and back. The PurePath client allows the analyst to see which transactions may have poor performance and take the appropriate action. The data is not aggregated. However, one poor user experience from 1000's of positive experiences can be quickly identified and analyzed.
Strobe is a sampling tool that is used for a short period of time and turned off. It is used either for performance troubleshooting or general performance tuning. It will go much deeper than PurePath and provide the data for instance, within a CICS region that is lacking in the PurePath reports. Strobe aggregates the data it collects over the measurement period. PurePath does not aggregate its data. Strobe also captures data only within the measured CICS region, if there is other activity in another CICS region that region will have to have an additional Strobe measurement initiated against it. PurePath will follow the transaction as it flows from region to region and report on that activity.