Things were so simple in the old days when it was easy to place vendors. New Relic was firmly and squarely focused on performance monitoring. It helps organizations gain visibility into how their applications and infrastructure are working so that they can make decisions or take actions based on performance.
Splunk was in an entirely different space and was all about allowing organizations to ingest a huge amount of their data in order to run analytics over that data. In doing so, Splunk promised to give organizations more visibility about what was going on.